Bach's home page

Johann Sebastian Bach was a prolific composer.
To set the text color of the H1 elements to red, you can write the
following CSS rules:
h1 { color: red }
A CSS rule consists of two main parts: selector ('h1') and declaration
('color: red'). In HTML, element names are case-insensitive so 'h1'
works just as well as 'H1'. The declaration has two parts: property
('color') and value ('red'). While the example above tries to
influence only one of the properties needed for rendering an HTML
document, it qualifies as a style sheet on its own. Combined with
other style sheets (one fundamental feature of CSS is that style
sheets are combined) it will determine the final presentation of the
document.
The HTML 4 specification defines how style sheet rules may be
specified for HTML documents: either within the HTML document, or via
an external style sheet. To put the style sheet into the document, use
the STYLE element:

Bach's home page

Bach's home page

Johann Sebastian Bach was a prolific composer.
For maximum flexibility, we recommend that authors specify external
style sheets; they may be changed without modifying the source HTML
document, and they may be shared among several documents. To link to
an external style sheet, you can use the LINK element:

Bach's home page

Bach's home page

Johann Sebastian Bach was a prolific composer.
The LINK element specifies:
* the type of link: to a "stylesheet".
* the location of the style sheet via the "href" attribute.
* the type of style sheet being linked: "text/css".
To show the close relationship between a style sheet and the
structured markup, we continue to use the STYLE element in this
tutorial. Let's add more colors:

Bach's home page

Bach's home page

Johann Sebastian Bach was a prolific composer.
The style sheet now contains four rules: the first two set the color
and background of the BODY element (it's a good idea to set the text
color and background color together), while the last two set the color
and the background of the H1 element. Since no color has been
specified for the P element, it will inherit the color from its parent
element, namely BODY. The H1 element is also a child element of BODY
but the second rule overrides the inherited value. In CSS there are
often such conflicts between different values, and this specification
describes how to resolve them.
CSS 2.1 has more than 90 properties, including 'color'. Let's look at
some of the others:

Bach's home page

Bach's home page

Johann Sebastian Bach was a prolific composer.
The first thing to notice is that several declarations are grouped
within a block enclosed by curly braces ({...}), and separated by
semicolons, though the last declaration may also be followed by a
semicolon.
The first declaration on the BODY element sets the font family to
"Gill Sans". If that font isn't available, the user agent (often
referred to as a "browser") will use the 'sans-serif' font family
which is one of five generic font families which all users agents
know. Child elements of BODY will inherit the value of the
'font-family' property.
The second declaration sets the font size of the BODY element to 12
points. The "point" unit is commonly used in print-based typography to
indicate font sizes and other length values. It's an example of an
absolute unit which does not scale relative to the environment.
The third declaration uses a relative unit which scales with regard to
its surroundings. The "em" unit refers to the font size of the
element. In this case the result is that the margins around the BODY
element are three times wider than the font size.
2.2 A brief CSS 2.1 tutorial for XML
This section is non-normative.
CSS can be used with any structured document format, for example with
applications of the eXtensible Markup Language [XML10]. In fact, XML
depends more on style sheets than HTML, since authors can make up
their own elements that user agents don't know how to display.
Here is a simple XML fragment:
Fredrick the Great meets BachJohann Nikolaus Forkel
One evening, just as he was getting his
flute ready and his
musicians were assembled, an officer brought him a list of
the strangers who had arrived.
To display this fragment in a document-like fashion, we must first
declare which elements are inline-level (i.e., do not cause line
breaks) and which are block-level (i.e., cause line breaks).
INSTRUMENT { display: inline }
ARTICLE, HEADLINE, AUTHOR, PARA { display: block }
The first rule declares INSTRUMENT to be inline and the second rule,
with its comma-separated list of selectors, declares all the other
elements to be block-level. Element names in XML are case-sensitive,
so a selector written in lowercase (e.g. 'instrument') is different
from uppercase (e.g. 'INSTRUMENT').
One way of linking a style sheet to an XML document is to use a
processing instruction:
Fredrick the Great meets BachJohann Nikolaus Forkel
One evening, just as he was getting his
flute ready and his
musicians were assembled, an officer brought him a list of
the strangers who had arrived.
A visual user agent could format the above example as:
Example rendering [D]
Notice that the word "flute" remains within the paragraph since it is
the content of the inline element INSTRUMENT.
Still, the text isn't formatted the way you would expect. For example,
the headline font size should be larger than then the rest of the
text, and you may want to display the author's name in italic:
INSTRUMENT { display: inline }
ARTICLE, HEADLINE, AUTHOR, PARA { display: block }
HEADLINE { font-size: 1.3em }
AUTHOR { font-style: italic }
ARTICLE, HEADLINE, AUTHOR, PARA { margin: 0.5em }
A visual user agent could format the above example as:
Example rendering [D]
Adding more rules to the style sheet will allow you to further
describe the presentation of the document.
2.3 The CSS 2.1 processing model
This section presents one possible model of how user agents that
support CSS work. This is only a conceptual model; real
implementations may vary.
In this model, a user agent processes a source by going through the
following steps:
1. Parse the source document and create a document tree.
2. Identify the target media type.
3. Retrieve all style sheets associated with the document that are
specified for the target media type.
4. Annotate every element of the document tree by assigning a single
value to every property that is applicable to the target media
type. Properties are assigned values according to the mechanisms
described in the section on cascading and inheritance.
Part of the calculation of values depends on the formatting
algorithm appropriate for the target media type. For example, if
the target medium is the screen, user agents apply the visual
formatting model.
5. From the annotated document tree, generate a formatting structure.
Often, the formatting structure closely resembles the document
tree, but it may also differ significantly, notably when authors
make use of pseudo-elements and generated content. First, the
formatting structure need not be "tree-shaped" at all -- the
nature of the structure depends on the implementation. Second, the
formatting structure may contain more or less information than the
document tree. For instance, if an element in the document tree
has a value of 'none' for the 'display' property, that element
will generate nothing in the formatting structure. A list element,
on the other hand, may generate more information in the formatting
structure: the list element's content and list style information
(e.g., a bullet image).
Note that the CSS user agent does not alter the document tree
during this phase. In particular, content generated due to style
sheets is not fed back to the document language processor (e.g.,
for reparsing).
6. Transfer the formatting structure to the target medium (e.g.,
print the results, display them on the screen, render them as
speech, etc.).
Step 1 lies outside the scope of this specification (see, for example,
[DOM]).
Steps 2-5 are addressed by the bulk of this specification.
Step 6 lies outside the scope of this specification.
2.3.1 The canvas
For all media, the term canvas describes "the space where the
formatting structure is rendered." The canvas is infinite for each
dimension of the space, but rendering generally occurs within a finite
region of the canvas, established by the user agent according to the
target medium. For instance, user agents rendering to a screen
generally impose a minimum width and choose an initial width based on
the dimensions of the viewport. User agents rendering to a page
generally impose width and height constraints. Aural user agents may
impose limits in audio space, but not in time.
2.3.2 CSS 2.1 addressing model
CSS 2.1 selectors and properties allow style sheets to refer to the
following parts of a document or user agent:
* Elements in the document tree and certain relationships between
them (see the section on selectors).
* Attributes of elements in the document tree, and values of those
attributes (see the section on attribute selectors).
* Some parts of element content (see the :first-line and
:first-letter pseudo-elements).
* Elements of the document tree when they are in a certain state
(see the section on pseudo-classes).
* Some aspects of the canvas where the document will be rendered.
* Some system information (see the section on user interface).
2.4 CSS design principles
This section is non-normative.
CSS 2.1, as CSS2 and CSS1 before it, is based on a set of design
principles:
* Forward and backward compatibility. CSS 2.1 user agents will be
able to understand CSS1 style sheets. CSS1 user agents will be
able to read CSS 2.1 style sheets and discard parts they don't
understand. Also, user agents with no CSS support will be able to
display style-enhanced documents. Of course, the stylistic
enhancements made possible by CSS will not be rendered, but all
content will be presented.
* Complementary to structured documents. Style sheets complement
structured documents (e.g., HTML and XML applications), providing
stylistic information for the marked-up text. It should be easy to
change the style sheet with little or no impact on the markup.
* Vendor, platform, and device independence. Style sheets enable
documents to remain vendor, platform, and device independent.
Style sheets themselves are also vendor and platform independent,
but CSS 2.1 allows you to target a style sheet for a group of
devices (e.g., printers).
* Maintainability. By pointing to style sheets from documents,
webmasters can simplify site maintenance and retain consistent
look and feel throughout the site. For example, if the
organization's background color changes, only one file needs to be
changed.
* Simplicity. CSS is a simple style language which is human readable
and writable. The CSS properties are kept independent of each
other to the largest extent possible and there is generally only
one way to achieve a certain effect.
* Network performance. CSS provides for compact encodings of how to
present content. Compared to images or audio files, which are
often used by authors to achieve certain rendering effects, style
sheets most often decrease the content size. Also, fewer network
connections have to be opened which further increases network
performance.
* Flexibility. CSS can be applied to content in several ways. The
key feature is the ability to cascade style information specified
in the default (user agent) style sheet, user style sheets, linked
style sheets, the document head, and in attributes for the
elements forming the document body.
* Richness. Providing authors with a rich set of rendering effects
increases the richness of the Web as a medium of expression.
Designers have been longing for functionality commonly found in
desktop publishing and slide-show applications. Some of the
requested rendering effects conflict with device independence, but
CSS 2.1 goes a long way toward granting designers their requests.
* Alternative language bindings. The set of CSS properties described
in this specification form a consistent formatting model for
visual and aural presentations. This formatting model can be
accessed through the CSS language, but bindings to other languages
are also possible. For example, a JavaScript program may
dynamically change the value of a certain element's 'color'
property.
* Accessibility. Several CSS features will make the Web more
accessible to users with disabilities:
+ Properties to control font appearance allow authors to
eliminate inaccessible bit-mapped text images.
+ Positioning properties allow authors to eliminate mark-up
tricks (e.g., invisible images) to force layout.
+ The semantics of !important rules mean that users with
particular presentation requirements can override the
author's style sheets.
+ The 'inherit' value for all properties improves cascading
generality and allows for easier and more consistent style
tuning.
+ Improved media support, including media groups and the
braille, embossed, and tty media types, will allow users and
authors to tailor pages to those devices.
Note. For more information about designing accessible documents
using CSS and HTML, see [WAI-PAGEAUTH].
_________________________________________________________________
_________________________________________________________________
3 Conformance: Requirements and Recommendations
Contents
* 3.1 Definitions
* 3.2 UA Conformance
* 3.3 Error conditions
* 3.4 The text/css content type
3.1 Definitions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 (see
[RFC2119]). However, for readability, these words do not appear in all
uppercase letters in this specification.
At times, this specification recommends good practice for authors and
user agents. These recommendations are not normative and conformance
with this specification does not depend on their realization. These
recommendations contain the expression "We recommend ...", "This
specification recommends ...", or some similar wording.
The fact that a feature is marked as deprecated (namely the 'aural'
keyword) or going to be deprecated in CSS3 (namely the system colors)
also has no influence on conformance. (For example, 'aural' is marked
as non-normative, so UAs do not need to support it; the system colors
are normative, so UAs must support them.)
All sections of this specification, including appendices, are
normative unless otherwise noted.
Examples and notes are not normative.
Example(s):
Examples usually have the word "example" near their start ("Example:",
"The following example...," "For example," etc.) and are shown in the
color maroon, like this paragraph.
Notes start with the word "Note," are indented and shown in green,
like this paragraph.
Figures are for illustration only, they are not reference renderings,
unless explicitly stated.
Style sheet
A set of statements that specify presentation of a document.
Style sheets may have three different origins: author, user,
and user agent. The interaction of these sources is described
in the section on cascading and inheritance.
Valid style sheet
The validity of a style sheet depends on the level of CSS used
for the style sheet. All valid CSS1 style sheets are valid
CSS 2.1 style sheets, but some changes from CSS1 mean that a
few CSS1 style sheets will have slightly different semantics in
CSS 2.1. Some features in CSS2 are not part of CSS 2.1, so not
all CSS2 style sheets are valid CSS 2.1 style sheets.
A valid CSS 2.1 style sheet must be written according to the
grammar of CSS 2.1. Furthermore, it must contain only at-rules,
property names, and property values defined in this
specification. An illegal (invalid) at-rule, property name, or
property value is one that is not valid.
Source document
The document to which one or more style sheets apply. This is
encoded in some language that represents the document as a tree
of elements. Each element consists of a name that identifies
the type of element, optionally a number of attributes, and a
(possibly empty) content. For example, the source document
could be an XML or SGML instance.
Document language
The encoding language of the source document (e.g., HTML, XHTML
or SVG). CSS is used to describe the presentation of document
languages and CSS does not change the underlying semantics of
the document languages.
Element
(An SGML term, see [ISO8879].) The primary syntactic constructs
of the document language. Most CSS style sheet rules use the
names of these elements (such as P, TABLE, and OL in HTML) to
specify how the elements should be rendered.
Replaced element
An element whose content is outside the scope of the CSS
formatting model, such as an image, embedded document, or
applet. For example, the content of the HTML IMG element is
often replaced by the image that its "src" attribute
designates. Replaced elements often have intrinsic dimensions:
an intrinsic width, an intrinsic height, and an intrinsic
ratio. For example, a bitmap image has an intrinsic width and
an intrinsic height specified in absolute units (from which the
intrinsic ratio can obviously be determined). On the other
hand, other documents may not have any intrinsic dimensions
(for example a blank HTML document).
User agents may consider a replaced element to not have any
intrinsic dimensions if it is believed that those dimensions
could leak sensitive information to a third party. For example,
if an HTML document changed intrinsic size depending on the
user's bank balance, then the UA might want to act as if that
resource had no intrinsic dimensions.
Intrinsic dimensions
The width and height as defined by the element itself, not
imposed by the surroundings. CSS does not define how the
intrinsic dimensions are found. In CSS 2.1 only replaced
elements can come with intrinsic dimensions.
Attribute
A value associated with an element, consisting of a name, and
an associated (textual) value.
Content
The content associated with an element in the source document.
Some elements have no content, in which case they are called
empty. The content of an element may include text, and it may
include a number of sub-elements, in which case the element is
called the parent of those sub-elements.
Ignore
This term has two slightly different meanings in this
specification. First, a CSS parser must follow certain rules
when it discovers unknown or illegal syntax in a style sheet.
The parser must then ignore certain parts of the style sheets.
The exact rules for what parts must be ignored is given in
these section: Declarations and properties, Rules for handling
parsing errors, Unsupported Values, or may be explained in the
text where the term "ignore" appears. Second, a user agent may
(and, in some cases must) disregard certain properties or
values in the style sheet even if the syntax is legal. For
example, table-column elements can't affect the font of the
column, so the font properties must be ignored.
Rendered content
The content of an element after the rendering that applies to
it according to the relevant style sheets has been applied. How
a replaced element's content is rendered is not defined by this
specification. Rendered content may also be alternate text for
an element (e.g., the value of the XHTML "alt" attribute), and
may include items inserted implicitly or explicitly by the
style sheet, such as bullets, numbering, etc.
Document tree
The tree of elements encoded in the source document. Each
element in this tree has exactly one parent, with the exception
of the root element, which has none.
Child
An element A is called the child of element B if and only if B
is the parent of A.
Descendant
An element A is called a descendant of an element B, if either
(1) A is a child of B, or (2) A is the child of some element C
that is a descendant of B.
Ancestor
An element A is called an ancestor of an element B, if and only
if B is a descendant of A.
Sibling
An element A is called a sibling of an element B, if and only
if B and A share the same parent element. Element A is a
preceding sibling if it comes before B in the document tree.
Element B is a following sibling if it comes after A in the
document tree.
Preceding element
An element A is called a preceding element of an element B, if
and only if (1) A is an ancestor of B or (2) A is a preceding
sibling of B.
Following element
An element A is called a following element of an element B, if
and only if B is a preceding element of A.
Author
An author is a person who writes documents and associated style
sheets. An authoring tool is a User Agent that generates style
sheets.
User
A user is a person who interacts with a user agent to view,
hear, or otherwise use a document and its associated style
sheet. The user may provide a personal style sheet that encodes
personal preferences.
User agent (UA)
A user agent is any program that interprets a document written
in the document language and applies associated style sheets
according to the terms of this specification. A user agent may
display a document, read it aloud, cause it to be printed,
convert it to another format, etc.
An HTML user agent is one that supports the HTML 2.x, HTML 3.x,
or HTML 4.x specifications. A user agent that supports XHTML
[XHTML], but not HTML (as listed in the previous sentence) is
not considered an HTML user agent for the purpose of
conformance with this specification.
Property
CSS defines a finite set of parameters, called properties, that
direct the rendering of a document. Each property has a name
(e.g., 'color', 'font' or border') and a value (e.g., 'red',
'12pt Times' or 'dotted'). Properties are attached to various
parts of the document and to the page on which the document is
to be displayed by the mechanisms of specificity, cascading and
inheritance (see the chapter on Assigning property values,
Cascading, and Inheritance).
Here is an example of a source document written in HTML:

My home page

My home page

Welcome to my home page! Let me tell you about my favorite
composers:

Elvis Costello

Johannes Brahms

Georges Brassens

This results in the following tree:
Sample document tree [D]
According to the definition of HTML 4, HEAD elements will be inferred
during parsing and become part of the document tree even if the "head"
tags are not in the document source. Similarly, the parser knows where
the P and LI elements end, even though there are no

and
tags in the source.
Documents written in XHTML (and other XML-based languages) behave
differently: there are no inferred elements and all elements must have
end tags.
3.2 UA Conformance
This section defines conformance with the CSS 2.1 specification only.
There may be other levels of CSS in the future that may require a user
agent to implement a different set of features in order to conform.
In general, the following points must be observed by a user agent
claiming conformance to this specification:
1. It must recognize one or more of the CSS 2.1 media types.
2. For each source document, it must attempt to retrieve all
associated style sheets that are appropriate for the recognized
media types. If it cannot retrieve all associated style sheets
(for instance, because of network errors), it must display the
document using those it can retrieve.
3. It must parse the style sheets according to this specification. In
particular, it must recognize all at-rules, blocks, declarations,
and selectors (see the grammar of CSS 2.1). If a user agent
encounters a property that applies for a supported media type, the
user agent must parse the value according to the property
definition. This means that the user agent must accept all valid
values and must ignore declarations with invalid values. User
agents must ignore rules that apply to unsupported media types.
4. For each element in a document tree, it must assign a value for
every property according to the property's definition and the
rules of cascading and inheritance.
5. If the source document comes with alternate style sheet sets (such
as with the "alternate" keyword in HTML 4 [HTML4]), the UA must
allow the user to select which style sheet set the UA should
apply.
6. The UA must allow the user to turn off the influence of author
style sheets.
Not every user agent must observe every point, however:
* An application that reads style sheets without rendering any
content (e.g., a CSS 2.1 validator) must respect points 1-3.
* An authoring tool is only required to output valid style sheets
* A user agent that renders a document with associated style sheets
must respect points 1-6 and render the document according to the
media-specific requirements set forth in this specification.
Values may be approximated when required by the user agent.
The inability of a user agent to implement part of this specification
due to the limitations of a particular device (e.g., a user agent
cannot render colors on a monochrome monitor or page) does not imply
non-conformance.
UAs must allow users to specify a file that contains the user style
sheet. UAs that run on devices without any means of writing or
specifying files are exempted from this requirement. Additionally, UAs
may offer other means to specify user preferences, for example through
a GUI.
CSS 2.1 does not define which properties apply to form controls and
frames, or how CSS can be used to style them. User agents may apply
CSS properties to these elements. Authors are recommended to treat
such support as experimental. A future level of CSS may specify this
further.
3.3 Error conditions
In general, this document specifies error handling behavior throughout
the specification. For example, see the rules for handling parsing
errors.
3.4 The text/css content type
CSS style sheets that exist in separate files are sent over the
Internet as a sequence of bytes accompanied by encoding information.
The structure of the transmission, termed a message entity, is defined
by RFC 2045 and RFC 2616 (see [RFC2045] and [RFC2616]). A message
entity with a content type of "text/css" represents an independent CSS
document. The "text/css" content type has been registered by RFC 2318
([RFC2318]).
_________________________________________________________________
_________________________________________________________________
4 Syntax and basic data types
Contents
* 4.1 Syntax
+ 4.1.1 Tokenization
+ 4.1.2 Keywords
o 4.1.2.1 Vendor-specific extensions
o 4.1.2.2 Informative Historical Notes
+ 4.1.3 Characters and case
+ 4.1.4 Statements
+ 4.1.5 At-rules
+ 4.1.6 Blocks
+ 4.1.7 Rule sets, declaration blocks, and selectors
+ 4.1.8 Declarations and properties
+ 4.1.9 Comments
* 4.2 Rules for handling parsing errors
* 4.3 Values
+ 4.3.1 Integers and real numbers
+ 4.3.2 Lengths
+ 4.3.3 Percentages
+ 4.3.4 URLs and URIs
+ 4.3.5 Counters
+ 4.3.6 Colors
+ 4.3.7 Strings
+ 4.3.8 Unsupported Values
* 4.4 CSS style sheet representation
+ 4.4.1 Referring to characters not represented in a character
encoding
4.1 Syntax
This section describes a grammar (and forward-compatible parsing
rules) common to any level of CSS (including CSS 2.1). Future updates
of CSS will adhere to this core syntax, although they may add
additional syntactic constraints.
These descriptions are normative. They are also complemented by the
normative grammar rules presented in Appendix G.
In this specification, the expressions "immediately before" or
"immediately after" mean with no intervening whitespace or comments.
4.1.1 Tokenization
All levels of CSS -- level 1, level 2, and any future levels -- use
the same core syntax. This allows UAs to parse (though not completely
understand) style sheets written in levels of CSS that didn't exist at
the time the UAs were created. Designers can use this feature to
create style sheets that work with older user agents, while also
exercising the possibilities of the latest levels of CSS.
At the lexical level, CSS style sheets consist of a sequence of
tokens. The list of tokens for CSS is as follows. The definitions use
Lex-style regular expressions. Octal codes refer to ISO 10646
([ISO10646]). As in Lex, in case of multiple matches, the longest
match determines the token.
Token Definition
_________________________________________________________________
IDENT {ident}
ATKEYWORD @{ident}
STRING {string}
INVALID {invalid}
HASH #{name}
NUMBER {num}
PERCENTAGE {num}%
DIMENSION {num}{ident}
URI url\({w}{string}{w}\)
|url\({w}([!#$%&*-~]|{nonascii}|{escape})*{w}\)
UNICODE-RANGE U\+[0-9a-f?]{1,6}(-[0-9a-f]{1,6})?
CDO
; ;
{ \{
} \}
( \(
) \)
[ \[
] \]
S [ \t\r\n\f]+
COMMENT \/\*[^*]*\*+([^/*][^*]*\*+)*\/
FUNCTION {ident}\(
INCLUDES ~=
DASHMATCH |=
DELIM any other character not matched by the above rules, and neither
a single nor a double quote
The macros in curly braces ({}) above are defined as follows:
Macro Definition
_________________________________________________________________
ident [-]?{nmstart}{nmchar}*
name {nmchar}+
nmstart [_a-z]|{nonascii}|{escape}
nonascii [^\0-\177]
unicode \\[0-9a-f]{1,6}(\r\n|[ \n\r\t\f])?
escape {unicode}|\\[^\n\r\f0-9a-f]
nmchar [_a-z0-9-]|{nonascii}|{escape}
num [0-9]+|[0-9]*\.[0-9]+
string {string1}|{string2}
string1 \"([^\n\r\f\\"]|\\{nl}|{escape})*\"
string2 \'([^\n\r\f\\']|\\{nl}|{escape})*\'
invalid {invalid1}|{invalid2}
invalid1 \"([^\n\r\f\\"]|\\{nl}|{escape})*
invalid2 \'([^\n\r\f\\']|\\{nl}|{escape})*
nl \n|\r\n|\r|\f
w [ \t\r\n\f]*
Below is the core syntax for CSS. The sections that follow describe
how to use it. Appendix G describes a more restrictive grammar that is
closer to the CSS level 2 language. Parts of style sheets that can be
parsed according to this grammar but not according to the grammar in
Appendix G are among the parts that will be ignored according to the
rules for handling parsing errors.
stylesheet : [ CDO | CDC | S | statement ]*;
statement : ruleset | at-rule;
at-rule : ATKEYWORD S* any* [ block | ';' S* ];
block : '{' S* [ any | block | ATKEYWORD S* | ';' S* ]* '}' S*;
ruleset : selector? '{' S* declaration? [ ';' S* declaration? ]* '}' S*;
selector : any+;
declaration : DELIM? property S* ':' S* value;
property : IDENT;
value : [ any | block | ATKEYWORD S* ]+;
any : [ IDENT | NUMBER | PERCENTAGE | DIMENSION | STRING
| DELIM | URI | HASH | UNICODE-RANGE | INCLUDES
| DASHMATCH | FUNCTION S* any* ')'
| '(' S* any* ')' | '[' S* any* ']' ] S*;
COMMENT tokens do not occur in the grammar (to keep it readable), but
any number of these tokens may appear anywhere between other tokens.
The token S in the grammar above stands for whitespace. Only the
characters "space" (U+0020), "tab" (U+0009), "line feed" (U+000A),
"carriage return" (U+000D), and "form feed" (U+000C) can occur in
whitespace. Other space-like characters, such as "em-space" (U+2003)
and "ideographic space" (U+3000), are never part of whitespace.
The meaning of input that cannot be tokenized or parsed is undefined
in CSS 2.1.
4.1.2 Keywords
Keywords have the form of identifiers. Keywords must not be placed
between quotes ("..." or '...'). Thus,
red
is a keyword, but
"red"
is not. (It is a string.) Other illegal examples:
Illegal example(s):
width: "auto";
border: "none";
background: "red";
4.1.2.1 Vendor-specific extensions
In CSS, identifiers may begin with '-' (dash) or '_' (underscore).
Keywords and property names beginning with -' or '_' are reserved for
vendor-specific extensions. Such vendor-specific extensions should
have one of the following formats:
'-' + vendor identifier + '-' + meaningful name
'_' + vendor identifier + '-' + meaningful name
Example(s):
For example, if XYZ organization added a property to describe the
color of the border on the East side of the display, they might call
it -xyz-border-east-color.
Other known examples:
-moz-box-sizing
-moz-border-radius
-wap-accesskey
An initial dash or underscore is guaranteed never to be used in a
property or keyword by any current or future level of CSS. Thus
typical CSS implementations may not recognize such properties and may
ignore them according to the rules for handling parsing errors.
However, because the initial dash or underscore is part of the
grammar, CSS 2.1 implementers should always be able to use a
CSS-conforming parser, whether or not they support any vendor-specific
extensions.
Authors should avoid vendor-specific extensions
4.1.2.2 Informative Historical Notes
This section is informative.
At the time of writing, the following prefixes are known to exist:
prefix organization
-ms- Microsoft
mso- Microsoft Office
-moz- Mozilla
-o- Opera Software
-atsc- Advanced Television Standards Committee
-wap- The WAP Forum
4.1.3 Characters and case
The following rules always hold:
* All CSS style sheets are case-insensitive, except for parts that
are not under the control of CSS. For example, the
case-sensitivity of values of the HTML attributes "id" and
"class", of font names, and of URIs lies outside the scope of this
specification. Note in particular that element names are
case-insensitive in HTML, but case-sensitive in XML.
* In CSS, identifiers (including element names, classes, and IDs in
selectors) can contain only the characters [a-z0-9] and ISO 10646
characters U+00A1 and higher, plus the hyphen (-) and the
underscore (_); they cannot start with a digit, or a hyphen
followed by a digit. Identifiers can also contain escaped
characters and any ISO 10646 character as a numeric code (see next
item). For instance, the identifier "B&W?" may be written as
"B\&W\?" or "B\26 W\3F".
Note that Unicode is code-by-code equivalent to ISO 10646 (see
[UNICODE] and [ISO10646]).
* In CSS 2.1, a backslash (\) character indicates three types of
character escapes.
First, inside a string, a backslash followed by a newline is
ignored (i.e., the string is deemed not to contain either the
backslash or the newline).
Second, it cancels the meaning of special CSS characters. Any
character (except a hexadecimal digit) can be escaped with a
backslash to remove its special meaning. For example, "\"" is a
string consisting of one double quote. Style sheet preprocessors
must not remove these backslashes from a style sheet since that
would change the style sheet's meaning.
Third, backslash escapes allow authors to refer to characters they
can't easily put in a document. In this case, the backslash is
followed by at most six hexadecimal digits (0..9A..F), which stand
for the ISO 10646 ([ISO10646]) character with that number, which
must not be zero. (It is undefined in CSS 2.1 what happens if a
style sheet does contain a character with Unicode codepoint zero.)
If a character in the range [0-9a-f] follows the hexadecimal
number, the end of the number needs to be made clear. There are
two ways to do that:
1. with a space (or other whitespace character): "\26 B" ("&B").
In this case, user agents should treat a "CR/LF" pair
(U+000D/U+000A) as a single whitespace character.
2. by providing exactly 6 hexadecimal digits: "\000026B" ("&B")
In fact, these two methods may be combined. Only one whitespace
character is ignored after a hexadecimal escape. Note that this
means that a "real" space after the escape sequence must itself
either be escaped or doubled.
If the number is outside the range allowed by Unicode (e.g.,
"\110000" is above the maximum 10FFFF allowed in current Unicode),
the UA may replace the escape with the "replacement character"
(U+FFFD). If the character is to be displayed, the UA should show
a visible symbol, such as a "missing character" glyph (cf. 15.2,
point 5).
* Note: Backslash escapes, where allowed, are always considered to
be part of an identifier or a string (i.e., "\7B" is not
punctuation, even though "{" is, and "\32" is allowed at the start
of a class name, even though "2" is not).
The identifier "te\st" is exactly the same identifier as "test".
4.1.4 Statements
A CSS style sheet, for any level of CSS, consists of a list of
statements (see the grammar above). There are two kinds of statements:
at-rules and rule sets. There may be whitespace around the statements.
4.1.5 At-rules
At-rules start with an at-keyword, an '@' character followed
immediately by an identifier (for example, '@import', '@page').
An at-rule consists of everything up to and including the next
semicolon (;) or the next block, whichever comes first.
CSS 2.1 user agents must ignore any '@import' rule that occurs inside
a block or after any valid statement other than an @charset or an
@import rule.
Illegal example(s):
Assume, for example, that a CSS 2.1 parser encounters this style
sheet:
@import "subs.css";
h1 { color: blue }
@import "list.css";
The second '@import' is illegal according to CSS 2.1. The CSS 2.1
parser ignores the whole at-rule, effectively reducing the style sheet
to:
@import "subs.css";
h1 { color: blue }
Illegal example(s):
In the following example, the second '@import' rule is invalid, since
it occurs inside a '@media' block.
@import "subs.css";
@media print {
@import "print-main.css";
body { font-size: 10pt }
}
h1 {color: blue }
Instead, to achieve the effect of only importing a style sheet for
'print' media, use the @import rule with media syntax, e.g.:
@import "subs.css";
@import "print-main.css" print;
@media print {
body { font-size: 10pt }
}
h1 {color: blue }
4.1.6 Blocks
A block starts with a left curly brace ({) and ends with the matching
right curly brace (}). In between there may be any tokens, except that
parentheses (( )), brackets ([ ]) and braces ({ }) must always occur
in matching pairs and may be nested. Single (') and double quotes (")
must also occur in matching pairs, and characters between them are
parsed as a string. See Tokenization above for the definition of a
string.
Illegal example(s):
Here is an example of a block. Note that the right brace between the
double quotes does not match the opening brace of the block, and that
the second single quote is an escaped character, and thus doesn't
match the first single quote:
{ causta: "}" + ({7} * '\'') }
Note that the above rule is not valid CSS 2.1, but it is still a block
as defined above.
4.1.7 Rule sets, declaration blocks, and selectors
A rule set (also called "rule") consists of a selector followed by a
declaration block.
A declaration block starts with a left curly brace ({) and ends with
the matching right curly brace (}). In between there must be a list of
zero or more semicolon-separated (;) declarations.
The selector (see also the section on selectors) consists of
everything up to (but not including) the first left curly brace ({). A
selector always goes together with a declaration block. When a user
agent can't parse the selector (i.e., it is not valid CSS 2.1), it
must ignore the declaration block as well.
CSS 2.1 gives a special meaning to the comma (,) in selectors.
However, since it is not known if the comma may acquire other meanings
in future updates of CSS, the whole statement should be ignored if
there is an error anywhere in the selector, even though the rest of
the selector may look reasonable in CSS 2.1.
Illegal example(s):
For example, since the "&" is not a valid token in a CSS 2.1 selector,
a CSS 2.1 user agent must ignore the whole second line, and not set
the color of H3 to red:
h1, h2 {color: green }
h3, h4 & h5 {color: red }
h6 {color: black }
Example(s):
Here is a more complex example. The first two pairs of curly braces
are inside a string, and do not mark the end of the selector. This is
a valid CSS 2.1 rule.
p[example="public class foo\
{\
private int x;\
\
foo(int x) {\
this.x = x;\
}\
\
}"] { color: red }
4.1.8 Declarations and properties
A declaration is either empty or consists of a property name, followed
by a colon (:), followed by a value. Around each of these there may be
whitespace.
Because of the way selectors work, multiple declarations for the same
selector may be organized into semicolon (;) separated groups.
Example(s):
Thus, the following rules:
h1 { font-weight: bold }
h1 { font-size: 12px }
h1 { line-height: 14px }
h1 { font-family: Helvetica }
h1 { font-variant: normal }
h1 { font-style: normal }
are equivalent to:
h1 {
font-weight: bold;
font-size: 12px;
line-height: 14px;
font-family: Helvetica;
font-variant: normal;
font-style: normal
}
A property name is an identifier. Any token may occur in the value.
Parentheses ("( )"), brackets ("[ ]"), braces ("{ }"), single quotes
(') and double quotes (") must come in matching pairs, and semicolons
not in strings must be escaped. Parentheses, brackets, and braces may
be nested. Inside the quotes, characters are parsed as a string.
The syntax of values is specified separately for each property, but in
any case, values are built from identifiers, strings, numbers,
lengths, percentages, URIs, colors, etc.
A user agent must ignore a declaration with an invalid property name
or an invalid value. Every CSS 2.1 property has its own syntactic and
semantic restrictions on the values it accepts.
Illegal example(s):
For example, assume a CSS 2.1 parser encounters this style sheet:
h1 { color: red; font-style: 12pt } /* Invalid value: 12pt */
p { color: blue; font-vendor: any; /* Invalid prop.: font-vendor */
font-variant: small-caps }
em em { font-style: normal }
The second declaration on the first line has an invalid value '12pt'.
The second declaration on the second line contains an undefined
property 'font-vendor'. The CSS 2.1 parser will ignore these
declarations, effectively reducing the style sheet to:
h1 { color: red; }
p { color: blue; font-variant: small-caps }
em em { font-style: normal }
4.1.9 Comments
Comments begin with the characters "/*" and end with the characters
"*/". They may occur anywhere between tokens, and their contents have
no influence on the rendering. Comments may not be nested.
CSS also allows the SGML comment delimiters ("") in
certain places defined by the grammar, but they do not delimit CSS
comments. They are permitted so that style rules appearing in an HTML
source document (in the STYLE element) may be hidden from pre-HTML 3.2
user agents. See the HTML 4 specification ([HTML4]) for more
information.
4.2 Rules for handling parsing errors
In some cases, user agents must ignore part of an illegal style sheet.
This specification defines ignore to mean that the user agent parses
the illegal part (in order to find its beginning and end), but
otherwise acts as if it had not been there. CSS 2.1 reserves for
future updates of CSS all property:value combinations and @-keywords
that do not contain an identifier beginning with dash or underscore.
Implementations must ignore such combinations (other than those
introduced by future updates of CSS).
To ensure that new properties and new values for existing properties
can be added in the future, user agents are required to obey the
following rules when they encounter the following scenarios:
* Unknown properties. User agents must ignore a declaration with an
unknown property. For example, if the style sheet is:
h1 { color: red; rotation: 70minutes }
the user agent will treat this as if the style sheet had been
h1 { color: red }
* Illegal values. User agents must ignore a declaration with an
illegal value. For example:
img { float: left } /* correct CSS 2.1 */
img { float: left here } /* "here" is not a value of 'float' */
img { background: "red" } /* keywords cannot be quoted */
img { border-width: 3 } /* a unit must be specified for length values */
A CSS 2.1 parser would honor the first rule and ignore the rest,
as if the style sheet had been:
img { float: left }
img { }
img { }
img { }
A user agent conforming to a future CSS specification may accept
one or more of the other rules as well.
* Malformed declarations. User agents must handle unexpected tokens
encountered while parsing a declaration by reading until the end
of the declaration, while observing the rules for matching pairs
of (), [], {}, "", and '', and correctly handling escapes. For
example, a malformed declaration may be missing a property, colon
(:) or value. The following are all equivalent:
p { color:green }
p { color:green; color } /* malformed declaration missing ':', value */
p { color:red; color; color:green } /* same with expected recovery */
p { color:green; color: } /* malformed declaration missing value */
p { color:red; color:; color:green } /* same with expected recovery */
p { color:green; color{;color:maroon} } /* unexpected tokens { } */
p { color:red; color{;color:maroon}; color:green } /* same with recovery */
* Invalid at-keywords. User agents must ignore an invalid at-keyword
together with everything following it, up to and including the
next semicolon (;) or block ({...}), whichever comes first. For
example, consider the following:
@three-dee {
@background-lighting {
azimuth: 30deg;
elevation: 190deg;
}
h1 { color: red }
}
h1 { color: blue }
The '@three-dee' at-rule is not part of CSS 2.1. Therefore, the
whole at-rule (up to, and including, the third right curly brace)
is ignored. A CSS 2.1 user agent ignores it, effectively reducing
the style sheet to:
h1 { color: blue }
Something inside an at-rule that is ignored because it is invalid,
such as an invalid declaration within an @media-rule, does not
make the entire at-rule invalid.
* Unexpected end of style sheet.
User agents must close all open constructs (for example: blocks,
parentheses, brackets, rules, strings, and comments) at the end of
the style sheet. For example:
@media screen {
p:before { content: 'Hello
would be treated the same as:
@media screen {
p:before { content: 'Hello'; }
}
in a conformant UA.
* Unexpected end of string.
User agents must close strings upon reaching the end of a line,
but then drop the construct (declaration or rule) in which the
string was found. For example:
p {
color: green;
font-family: 'Courier New Times
color: red;
color: green;
}
...would be treated the same as:
p { color: green; color: green; }
...because the second declaration (from 'font-family' to the
semicolon after 'color: red') is invalid and is dropped.
* See also Rule sets, declaration blocks, and selectors for parsing
rules for declaration blocks.
4.3 Values
4.3.1 Integers and real numbers
Some value types may have integer values (denoted by ) or
real number values (denoted by ). Real numbers and integers
are specified in decimal notation only. An consists of one
or more digits "0" to "9". A can either be an , or
it can be zero or more digits followed by a dot (.) followed by one or
more digits. Both integers and real numbers may be preceded by a "-"
or "+" to indicate the sign. -0 is equivalent to 0 and is not a
negative number.
Note that many properties that allow an integer or real number as a
value actually restrict the value to some range, often to a
non-negative value.
4.3.2 Lengths
Lengths refer to horizontal or vertical measurements.
The format of a length value (denoted by in this
specification) is a (with or without a decimal point)
immediately followed by a unit identifier (e.g., px, em, etc.). After
a zero length, the unit identifier is optional.
Some properties allow negative length values, but this may complicate
the formatting model and there may be implementation-specific limits.
If a negative length value cannot be supported, it should be converted
to the nearest value that can be supported.
If a negative length value is set on a property that does not allow
negative length values, the declaration is ignored.
There are two types of length units: relative and absolute. Relative
length units specify a length relative to another length property.
Style sheets that use relative units will more easily scale from one
medium to another (e.g., from a computer display to a laser printer).
Relative units are:
* em: the 'font-size' of the relevant font
* ex: the 'x-height' of the relevant font
* px: pixels, relative to the viewing device
Example(s):
h1 { margin: 0.5em } /* em */
h1 { margin: 1ex } /* ex */
p { font-size: 12px } /* px */
The 'em' unit is equal to the computed value of the 'font-size'
property of the element on which it is used. The exception is when
'em' occurs in the value of the 'font-size' property itself, in which
case it refers to the font size of the parent element. It may be used
for vertical or horizontal measurement. (This unit is also sometimes
called the quad-width in typographic texts.)
The 'ex' unit is defined by the element's first available font. The
'x-height' is so called because it is often equal to the height of the
lowercase "x". However, an 'ex' is defined even for fonts that don't
contain an "x".
The x-height of a font can be found in different ways. Some fonts
contain reliable metrics for the x-height. If reliable font metrics
are not available, UAs may determine the x-height from the height of a
lowercase glyph. One possible heuristics is to look at how far the
glyph for the lowercase "o" extends below the baseline, and subtract
that value from the top of its bounding box. In the cases where it is
impossible or impractical to determine the x-height, a value of 0.5em
should be used.
Example(s):
The rule:
h1 { line-height: 1.2em }
means that the line height of "h1" elements will be 20% greater than
the font size of the "h1" elements. On the other hand:
h1 { font-size: 1.2em }
means that the font-size of "h1" elements will be 20% greater than the
font size inherited by "h1" elements.
When specified for the root of the document tree (e.g., "HTML" in
HTML), 'em' and 'ex' refer to the property's initial value.
Pixel units are relative to the resolution of the viewing device,
i.e., most often a computer display. If the pixel density of the
output device is very different from that of a typical computer
display, the user agent should rescale pixel values. It is recommended
that the reference pixel be the visual angle of one pixel on a device
with a pixel density of 96dpi and a distance from the reader of an
arm's length. For a nominal arm's length of 28 inches, the visual
angle is therefore about 0.0213 degrees.
For reading at arm's length, 1px thus corresponds to about 0.26 mm
(1/96 inch). When printed on a laser printer, meant for reading at a
little less than arm's length (55 cm, 21 inches), 1px is about
0.20 mm. On a 300 dots-per-inch (dpi) printer, that may be rounded up
to 3 dots (0.25 mm); on a 600 dpi printer, it can be rounded to 5
dots.
The two images below illustrate the effect of viewing distance on the
size of a pixel and the effect of a device's resolution. In the first
image, a reading distance of 71 cm (28 inch) results in a px of
0.26 mm, while a reading distance of 3.5 m (12 feet) requires a px of
1.3 mm.
Showing that pixels must become larger if the viewing distance
increases [D]
In the second image, an area of 1px by 1px is covered by a single dot
in a low-resolution device (a computer screen), while the same area is
covered by 16 dots in a higher resolution device (such as a 400 dpi
laser printer).
Showing that more device pixels (dots) are needed to cover a 1px by
1px area on a high-resolution device than on a low-res one [D]
Child elements do not inherit the relative values specified for their
parent; they inherit the computed values.
Example(s):
In the following rules, the computed 'text-indent' value of "h1"
elements will be 36px, not 45px, if "h1" is a child of the "body"
element.
body {
font-size: 12px;
text-indent: 3em; /* i.e., 36px */
}
h1 { font-size: 15px }
Absolute length units are only useful when the physical properties of
the output medium are known. The absolute units are:
* in: inches -- 1 inch is equal to 2.54 centimeters.
* cm: centimeters
* mm: millimeters
* pt: points -- the points used by CSS 2.1 are equal to 1/72nd of an
inch.
* pc: picas -- 1 pica is equal to 12 points.
Example(s):
h1 { margin: 0.5in } /* inches */
h2 { line-height: 3cm } /* centimeters */
h3 { word-spacing: 4mm } /* millimeters */
h4 { font-size: 12pt } /* points */
h4 { font-size: 1pc } /* picas */
In cases where the used length cannot be supported, user agents must
approximate it in the actual value.
4.3.3 Percentages
The format of a percentage value (denoted by in this
specification) is a immediately followed by '%'.
Percentage values are always relative to another value, for example a
length. Each property that allows percentages also defines the value
to which the percentage refers. The value may be that of another
property for the same element, a property for an ancestor element, or
a value of the formatting context (e.g., the width of a containing
block). When a percentage value is set for a property of the root
element and the percentage is defined as referring to the inherited
value of some property, the resultant value is the percentage times
the initial value of that property.
Example(s):
Since child elements (generally) inherit the computed values of their
parent, in the following example, the children of the P element will
inherit a value of 12px for 'line-height', not the percentage value
(120%):
p { font-size: 10px }
p { line-height: 120% } /* 120% of 'font-size' */
4.3.4 URLs and URIs
URI values (Uniform Resource Identifiers, see [RFC3986], which
includes URLs, URNs, etc) in this specification are denoted by .
The functional notation used to designate URIs in property values is
"url()", as in:
Example(s):
body { background: url("http://www.example.com/pinkish.png") }
The format of a URI value is 'url(' followed by optional whitespace
followed by an optional single quote (') or double quote (") character
followed by the URI itself, followed by an optional single quote (')
or double quote (") character followed by optional whitespace followed
by ')'. The two quote characters must be the same.
Example(s):
An example without quotes:
li { list-style: url(http://www.example.com/redball.png) disc }
Some characters appearing in an unquoted URI, such as parentheses,
commas, whitespace characters, single quotes (') and double quotes
("), must be escaped with a backslash so that the resulting URI value
is a URI token: '\(', '\)', '\,'.
Depending on the type of URI, it might also be possible to write the
above characters as URI-escapes (where "(" = %28, ")" = %29, etc.) as
described in [RFC3986].
In order to create modular style sheets that are not dependent on the
absolute location of a resource, authors may use relative URIs.
Relative URIs (as defined in [RFC3986]) are resolved to full URIs
using a base URI. RFC 3986, section 5, defines the normative algorithm
for this process. For CSS style sheets, the base URI is that of the
style sheet, not that of the source document.
Example(s):
For example, suppose the following rule:
body { background: url("yellow") }
is located in a style sheet designated by the URI:
http://www.example.org/style/basic.css
The background of the source document's BODY will be tiled with
whatever image is described by the resource designated by the URI
http://www.example.org/style/yellow
User agents may vary in how they handle invalid URIs or URIs that
designate unavailable or inapplicable resources.
4.3.5 Counters
Counters are denoted by identifiers (see the 'counter-increment' and
'counter-reset' properties). To refer to the value of a counter, the
notation 'counter()' or 'counter(,
)', with optional whitespace separating the tokens,
is used. The default style is 'decimal'.
To refer to a sequence of nested counters of the same name, the
notation is 'counters(, )' or
'counters(, , )' with optional
whitespace separating the tokens.
See "Nested counters and scope" in the chapter on generated content
for how user agents must determine the value or values of the counter.
See the definition of counter values of the 'content' property for how
it must convert these values to a string.
In CSS 2.1, the values of counters can only be referred to from the
'content' property. Note that 'none' is a possible
: 'counter(x, none)' yields an empty string.
Example(s):
Here is a style sheet that numbers paragraphs (p) for each chapter
(h1). The paragraphs are numbered with roman numerals, followed by a
period and a space:
p {counter-increment: par-num}
h1 {counter-reset: par-num}
p:before {content: counter(par-num, upper-roman) ". "}
4.3.6 Colors
A is either a keyword or a numerical RGB specification.
The list of color keywords is: aqua, black, blue, fuchsia, gray,
green, lime, maroon, navy, olive, orange, purple, red, silver, teal,
white, and yellow. These 17 colors have the following values:
maroon #800000 red #ff0000 orange #ffA500 yellow #ffff00 olive #808000
purple #800080 fuchsia #ff00ff white #ffffff lime #00ff00 green
#008000
navy #000080 blue #0000ff aqua #00ffff teal #008080
black #000000 silver #c0c0c0 gray #808080
In addition to these color keywords, users may specify keywords that
correspond to the colors used by certain objects in the user's
environment. Please consult the section on system colors for more
information.
Example(s):
body {color: black; background: white }
h1 { color: maroon }
h2 { color: olive }
The RGB color model is used in numerical color specifications. These
examples all specify the same color:
Example(s):
em { color: #f00 } /* #rgb */
em { color: #ff0000 } /* #rrggbb */
em { color: rgb(255,0,0) }
em { color: rgb(100%, 0%, 0%) }
The format of an RGB value in hexadecimal notation is a '#'
immediately followed by either three or six hexadecimal characters.
The three-digit RGB notation (#rgb) is converted into six-digit form
(#rrggbb) by replicating digits, not by adding zeros. For example,
#fb0 expands to #ffbb00. This ensures that white (#ffffff) can be
specified with the short notation (#fff) and removes any dependencies
on the color depth of the display.
The format of an RGB value in the functional notation is 'rgb('
followed by a comma-separated list of three numerical values (either
three integer values or three percentage values) followed by ')'. The
integer value 255 corresponds to 100%, and to F or FF in the
hexadecimal notation: rgb(255,255,255) = rgb(100%,100%,100%) = #FFF.
Whitespace characters are allowed around the numerical values.
All RGB colors are specified in the sRGB color space (see [SRGB]).
User agents may vary in the fidelity with which they represent these
colors, but using sRGB provides an unambiguous and objectively
measurable definition of what the color should be, which can be
related to international standards (see [COLORIMETRY]).
Conforming user agents may limit their color-displaying efforts to
performing a gamma-correction on them. sRGB specifies a display gamma
of 2.2 under specified viewing conditions. User agents should adjust
the colors given in CSS such that, in combination with an output
device's "natural" display gamma, an effective display gamma of 2.2 is
produced. See the section on gamma correction for further details.
Note that only colors specified in CSS are affected; e.g., images are
expected to carry their own color information.
Values outside the device gamut should be clipped or mapped into the
gamut when the gamut is known: the red, green, and blue values must be
changed to fall within the range supported by the device. Users agents
may perform higher quality mapping of colors from one gamut to
another. For a typical CRT monitor, whose device gamut is the same as
sRGB, the four rules below are equivalent:
Example(s):
em { color: rgb(255,0,0) } /* integer range 0 - 255 */
em { color: rgb(300,0,0) } /* clipped to rgb(255,0,0) */
em { color: rgb(255,-10,0) } /* clipped to rgb(255,0,0) */
em { color: rgb(110%, 0%, 0%) } /* clipped to rgb(100%,0%,0%) */
Other devices, such as printers, have different gamuts than sRGB; some
colors outside the 0..255 sRGB range will be representable (inside the
device gamut), while other colors inside the 0..255 sRGB range will be
outside the device gamut and will thus be mapped.
Note. Mapping or clipping of color values should be done to the actual
device gamut if known (which may be larger or smaller than 0..255).
4.3.7 Strings
Strings can either be written with double quotes or with single
quotes. Double quotes cannot occur inside double quotes, unless
escaped (e.g., as '\"' or as '\22'). Analogously for single quotes
(e.g., "\'" or "\27").
Example(s):
"this is a 'string'"
"this is a \"string\""
'this is a "string"'
'this is a \'string\''
A string cannot directly contain a newline. To include a newline in a
string, use an escape representing the line feed character in
ISO-10646 (U+000A), such as "\A" or "\00000a". This character
represents the generic notion of "newline" in CSS. See the 'content'
property for an example.
It is possible to break strings over several lines, for esthetic or
other reasons, but in such a case the newline itself has to be escaped
with a backslash (\). For instance, the following two selectors are
exactly the same:
Example(s):
a[title="a not s\
o very long title"] {/*...*/}
a[title="a not so very long title"] {/*...*/}
4.3.8 Unsupported Values
If a UA does not support a particular value, it should ignore that
value when parsing style sheets, as if that value was an illegal
value. For example:
Example(s):
h3 {
display: inline;
display: run-in;
}
A UA that supports the 'run-in' value for the 'display' property will
accept the first display declaration and then "write over" that value
with the second display declaration. A UA that does not support the
'run-in' value will process the first display declaration and ignore
the second display declaration.
4.4 CSS style sheet representation
A CSS style sheet is a sequence of characters from the Universal
Character Set (see [ISO10646]). For transmission and storage, these
characters must be encoded by a character encoding that supports the
set of characters available in US-ASCII (e.g., UTF-8, ISO 8859-x,
SHIFT JIS, etc.). For a good introduction to character sets and
character encodings, please consult the HTML 4 specification ([HTML4],
chapter 5). See also the XML 1.0 specification ([XML10], sections 2.2
and 4.3.3, and Appendix F).
When a style sheet is embedded in another document, such as in the
STYLE element or "style" attribute of HTML, the style sheet shares the
character encoding of the whole document.
When a style sheet resides in a separate file, user agents must
observe the following priorities when determining a style sheet's
character encoding (from highest priority to lowest):
1. An HTTP "charset" parameter in a "Content-Type" field (or similar
parameters in other protocols)
2. BOM and/or @charset (see below)
3. or other metadata from the linking mechanism (if
any)
4. charset of referring style sheet or document (if any)
5. Assume UTF-8
Authors using an @charset rule must place the rule at the very
beginning of the style sheet, preceded by no characters. (If a byte
order mark is appropriate for the encoding used, it may precede the
@charset rule.)
After "@charset", authors specify the name of a character encoding (in
quotes). For example:
@charset "ISO-8859-1";
@charset must be written literally, i.e., the 10 characters '@charset
"' (lowercase, no backslash escapes), followed by the encoding name,
followed by '";'.
The name must be a charset name as described in the IANA registry. See
[CHARSETS] for a complete list of charsets. Authors should use the
charset names marked as "preferred MIME name" in the IANA registry.
User agents must support at least the UTF-8 encoding.
User agents must ignore any @charset rule not at the beginning of the
style sheet. When user agents detect the character encoding using the
BOM and/or the @charset rule, they should follow the following rules:
* Except as specified in these rules, all @charset rules are
ignored.
* The encoding is detected based on the stream of bytes that begins
the style sheet. The following table gives a set of possibilities
for initial byte sequences (written in hexadecimal). The first row
that matches the beginning of the style sheet gives the result of
encoding detection based on the BOM and/or @charset rule. If no
rows match, the encoding cannot be detected based on the BOM
and/or @charset rule. The notation (...)* refers to repetition for
which the best match is the one that repeats as few times as
possible. The bytes marked "XX" are those used to determine the
name of the encoding, by treating them, in the order given, as a
sequence of ASCII characters. Bytes marked "YY" are similar, but
need to be transcoded into ASCII as noted. User agents may ignore
entries in the table if they do not support any encodings relevant
to the entry.
Initial Bytes Result
EF BB BF 40 63 68 61 72 73 65 74 20 22 (XX)* 22 3B as specified
EF BB BF UTF-8
40 63 68 61 72 73 65 74 20 22 (XX)* 22 3B as specified
FE FF 00 40 00 63 00 68 00 61 00 72 00 73 00 65 00 74 00 20 00 22 (00
XX)* 00 22 00 3B as specified (with BE endianness if not specified)
00 40 00 63 00 68 00 61 00 72 00 73 00 65 00 74 00 20 00 22 (00 XX)*
00 22 00 3B as specified (with BE endianness if not specified)
FF FE 40 00 63 00 68 00 61 00 72 00 73 00 65 00 74 00 20 00 22 00 (XX
00)* 22 00 3B 00 as specified (with LE endianness if not specified)
40 00 63 00 68 00 61 00 72 00 73 00 65 00 74 00 20 00 22 00 (XX 00)*
22 00 3B 00 as specified (with LE endianness if not specified)
00 00 FE FF 00 00 00 40 00 00 00 63 00 00 00 68 00 00 00 61 00 00 00
72 00 00 00 73 00 00 00 65 00 00 00 74 00 00 00 20 00 00 00 22 (00 00
00 XX)* 00 00 00 22 00 00 00 3B as specified (with BE endianness if
not specified)
00 00 00 40 00 00 00 63 00 00 00 68 00 00 00 61 00 00 00 72 00 00 00
73 00 00 00 65 00 00 00 74 00 00 00 20 00 00 00 22 (00 00 00 XX)* 00
00 00 22 00 00 00 3B as specified (with BE endianness if not
specified)
00 00 FF FE 00 00 40 00 00 00 63 00 00 00 68 00 00 00 61 00 00 00 72
00 00 00 73 00 00 00 65 00 00 00 74 00 00 00 20 00 00 00 22 00 (00 00
XX 00)* 00 00 22 00 00 00 3B 00 as specified (with 2143 endianness if
not specified)
00 00 40 00 00 00 63 00 00 00 68 00 00 00 61 00 00 00 72 00 00 00 73
00 00 00 65 00 00 00 74 00 00 00 20 00 00 00 22 00 (00 00 XX 00)* 00
00 22 00 00 00 3B 00 as specified (with 2143 endianness if not
specified)
FE FF 00 00 00 40 00 00 00 63 00 00 00 68 00 00 00 61 00 00 00 72 00
00 00 73 00 00 00 65 00 00 00 74 00 00 00 20 00 00 00 22 00 00 (00 XX
00 00)* 00 22 00 00 00 3B 00 00 as specified (with 3412 endianness if
not specified)
00 40 00 00 00 63 00 00 00 68 00 00 00 61 00 00 00 72 00 00 00 73 00
00 00 65 00 00 00 74 00 00 00 20 00 00 00 22 00 00 (00 XX 00 00)* 00
22 00 00 00 3B 00 00 as specified (with 3412 endianness if not
specified)
FF FE 00 00 40 00 00 00 63 00 00 00 68 00 00 00 61 00 00 00 72 00 00
00 73 00 00 00 65 00 00 00 74 00 00 00 20 00 00 00 22 00 00 00 (XX 00
00 00)* 22 00 00 00 3B 00 00 00 as specified (with LE endianness if
not specified)
40 00 00 00 63 00 00 00 68 00 00 00 61 00 00 00 72 00 00 00 73 00 00
00 65 00 00 00 74 00 00 00 20 00 00 00 22 00 00 00 (XX 00 00 00)* 22
00 00 00 3B 00 00 00 as specified (with LE endianness if not
specified)
00 00 FE FF UTF-32-BE
FF FE 00 00 UTF-32-LE
00 00 FF FE UTF-32-2143
FE FF 00 00 UTF-32-3412
FE FF UTF-16-BE
FF FE UTF-16-LE
7C 83 88 81 99 A2 85 A3 40 7F (YY)* 7F 5E as specified, transcoded
from EBCDIC to ASCII
AE 83 88 81 99 A2 85 A3 40 FC (YY)* FC 5E as specified, transcoded
from IBM1026 to ASCII
00 63 68 61 72 73 65 74 20 22 (YY)* 22 3B as specified, transcoded
from GSM 03.38 to ASCII
analogous patterns User agents may support additional, analogous,
patterns if they support encodings that are not handled by the
patterns here
* If the encoding is detected based on one of the entries in the
table above marked "as specified", the user agent ignores the
style sheet if it does not parse an appropriate @charset rule at
the beginning of the stream of characters resulting from decoding
in the chosen @charset. This ensures that:
+ @charset rules should only function if they are in the
encoding of the style sheet,
+ byte order marks are ignored only in encodings that support a
byte order mark, and
+ encoding names cannot contain newlines.
User agents must ignore style sheets in unknown encodings.
4.4.1 Referring to characters not represented in a character encoding
A style sheet may have to refer to characters that cannot be
represented in the current character encoding. These characters must
be written as escaped references to ISO 10646 characters. These
escapes serve the same purpose as numeric character references in HTML
or XML documents (see [HTML4], chapters 5 and 25).
The character escape mechanism should be used when only a few
characters must be represented this way. If most of a style sheet
requires escaping, authors should encode it with a more appropriate
encoding (e.g., if the style sheet contains a lot of Greek characters,
authors might use "ISO-8859-7" or "UTF-8").
Intermediate processors using a different character encoding may
translate these escaped sequences into byte sequences of that
encoding. Intermediate processors must not, on the other hand, alter
escape sequences that cancel the special meaning of an ASCII
character.
Conforming user agents must correctly map to ISO-10646 all characters
in any character encodings that they recognize (or they must behave as
if they did).
For example, a style sheet transmitted as ISO-8859-1 (Latin-1) cannot
contain Greek letters directly: "kouro*s" (Greek: "kouros") has to be
written as "\3BA\3BF\3C5\3C1\3BF\3C2".
Note. In HTML 4, numeric character references are interpreted in
"style" attribute values but not in the content of the STYLE element.
Because of this asymmetry, we recommend that authors use the CSS
character escape mechanism rather than numeric character references
for both the "style" attribute and the STYLE element. For example, we
recommend:
...
rather than:
...
_________________________________________________________________
_________________________________________________________________
5 Selectors
Contents
* 5.1 Pattern matching
* 5.2 Selector syntax
+ 5.2.1 Grouping
* 5.3 Universal selector
* 5.4 Type selectors
* 5.5 Descendant selectors
* 5.6 Child selectors
* 5.7 Adjacent sibling selectors
* 5.8 Attribute selectors
+ 5.8.1 Matching attributes and attribute values
+ 5.8.2 Default attribute values in DTDs
+ 5.8.3 Class selectors
* 5.9 ID selectors
* 5.10 Pseudo-elements and pseudo-classes
* 5.11 Pseudo-classes
+ 5.11.1 :first-child pseudo-class
+ 5.11.2 The link pseudo-classes: :link and :visited
+ 5.11.3 The dynamic pseudo-classes: :hover, :active, and
:focus
+ 5.11.4 The language pseudo-class: :lang
* 5.12 Pseudo-elements
+ 5.12.1 The :first-line pseudo-element
+ 5.12.2 The :first-letter pseudo-element
+ 5.12.3 The :before and :after pseudo-elements
5.1 Pattern matching
In CSS, pattern matching rules determine which style rules apply to
elements in the document tree. These patterns, called selectors, may
range from simple element names to rich contextual patterns. If all
conditions in the pattern are true for a certain element, the selector
matches the element.
The case-sensitivity of document language element names in selectors
depends on the document language. For example, in HTML, element names
are case-insensitive, but in XML they are case-sensitive.
The following table summarizes CSS 2.1 selector syntax:
Pattern Meaning Described in section
* Matches any element. Universal selector
E Matches any E element (i.e., an element of type E). Type selectors
E F Matches any F element that is a descendant of an E element.
Descendant selectors
E > F Matches any F element that is a child of an element E. Child
selectors
E:first-child Matches element E when E is the first child of its
parent. The :first-child pseudo-class
E:link
E:visited Matches element E if E is the source anchor of a hyperlink
of which the target is not yet visited (:link) or already visited
(:visited). The link pseudo-classes
E:active
E:hover
E:focus Matches E during certain user actions. The dynamic
pseudo-classes
E:lang(c) Matches element of type E if it is in (human) language c
(the document language specifies how language is determined). The
:lang() pseudo-class
E + F Matches any F element immediately preceded by a sibling element
E. Adjacent selectors
E[foo] Matches any E element with the "foo" attribute set (whatever
the value). Attribute selectors
E[foo="warning"] Matches any E element whose "foo" attribute value is
exactly equal to "warning". Attribute selectors
E[foo~="warning"] Matches any E element whose "foo" attribute value is
a list of space-separated values, one of which is exactly equal to
"warning". Attribute selectors
E[lang|="en"] Matches any E element whose "lang" attribute has a
hyphen-separated list of values beginning (from the left) with "en".
Attribute selectors
DIV.warning Language specific. (In HTML, the same as
DIV[class~="warning"].) Class selectors
E#myid Matches any E element with ID equal to "myid". ID selectors
5.2 Selector syntax
A simple selector is either a type selector or universal selector
followed immediately by zero or more attribute selectors, ID
selectors, or pseudo-classes, in any order. The simple selector
matches if all of its components match.
Note: the terminology used here in CSS 2.1 is different from what is
used in CSS3. For example, a "simple selector" refers to a smaller
part of a selector in CSS3 than in CSS 2.1. See the CSS3 Selectors
module [CSS3SEL].
A selector is a chain of one or more simple selectors separated by
combinators. Combinators are: whitespace, ">", and "+". Whitespace may
appear between a combinator and the simple selectors around it.
The elements of the document tree that match a selector are called
subjects of the selector. A selector consisting of a single simple
selector matches any element satisfying its requirements. Prepending a
simple selector and combinator to a chain imposes additional matching
constraints, so the subjects of a selector are always a subset of the
elements matching the last simple selector.
One pseudo-element may be appended to the last simple selector in a
chain, in which case the style information applies to a subpart of
each subject.
5.2.1 Grouping
When several selectors share the same declarations, they may be
grouped into a comma-separated list.
Example(s):
In this example, we condense three rules with identical declarations
into one. Thus,
h1 { font-family: sans-serif }
h2 { font-family: sans-serif }
h3 { font-family: sans-serif }
is equivalent to:
h1, h2, h3 { font-family: sans-serif }
CSS offers other "shorthand" mechanisms as well, including multiple
declarations and shorthand properties.
5.3 Universal selector
The universal selector, written "*", matches the name of any element
type. It matches any single element in the document tree.
If the universal selector is not the only component of a simple
selector, the "*" may be omitted. For example:
* *[lang=fr] and [lang=fr] are equivalent.
* *.warning and .warning are equivalent.
* *#myid and #myid are equivalent.
5.4 Type selectors
A type selector matches the name of a document language element type.
A type selector matches every instance of the element type in the
document tree.
Example(s):
The following rule matches all H1 elements in the document tree:
h1 { font-family: sans-serif }
5.5 Descendant selectors
At times, authors may want selectors to match an element that is the
descendant of another element in the document tree (e.g., "Match those
EM elements that are contained by an H1 element"). Descendant
selectors express such a relationship in a pattern. A descendant
selector is made up of two or more selectors separated by whitespace.
A descendant selector of the form "A B" matches when an element B is
an arbitrary descendant of some ancestor element A.
Example(s):
For example, consider the following rules:
h1 { color: red }
em { color: red }
Although the intention of these rules is to add emphasis to text by
changing its color, the effect will be lost in a case such as:

This headline is very important

We address this case by supplementing the previous rules with a rule
that sets the text color to blue whenever an EM occurs anywhere within
an H1:
h1 { color: red }
em { color: red }
h1 em { color: blue }
The third rule will match the EM in the following fragment:

This headline
is very important

Example(s):
The following selector:
div * p
matches a P element that is a grandchild or later descendant of a DIV
element. Note the whitespace on either side of the "*" is not part of
the universal selector; the whitespace is a combinator indicating that
the DIV must be the ancestor of some element, and that that element
must be an ancestor of the P.
Example(s):
The selector in the following rule, which combines descendant and
attribute selectors, matches any element that (1) has the "href"
attribute set and (2) is inside a P that is itself inside a DIV:
div p *[href]
5.6 Child selectors
A child selector matches when an element is the child of some element.
A child selector is made up of two or more selectors separated by ">".
Example(s):
The following rule sets the style of all P elements that are children
of BODY:
body > P { line-height: 1.3 }
Example(s):
The following example combines descendant selectors and child
selectors:
div ol>li p
It matches a P element that is a descendant of an LI; the LI element
must be the child of an OL element; the OL element must be a
descendant of a DIV. Notice that the optional whitespace around the
">" combinator has been left out.
For information on selecting the first child of an element, please see
the section on the :first-child pseudo-class below.
5.7 Adjacent sibling selectors
Adjacent sibling selectors have the following syntax: E1 + E2, where
E2 is the subject of the selector. The selector matches if E1 and E2
share the same parent in the document tree and E1 immediately precedes
E2, ignoring non-element nodes (such as text nodes and comments).
Example(s):
Thus, the following rule states that when a P element immediately
follows a MATH element, it should not be indented:
math + p { text-indent: 0 }
The next example reduces the vertical space separating an H1 and an H2
that immediately follows it:
h1 + h2 { margin-top: -5mm }
Example(s):
The following rule is similar to the one in the previous example,
except that it adds a class selector. Thus, special formatting only
occurs when H1 has class="opener":
h1.opener + h2 { margin-top: -5mm }
5.8 Attribute selectors
CSS 2.1 allows authors to specify rules that match elements which have
certain attributes defined in the source document.
5.8.1 Matching attributes and attribute values
Attribute selectors may match in four ways:
[att]
Match when the element sets the "att" attribute, whatever the
value of the attribute.
[att=val]
Match when the element's "att" attribute value is exactly
"val".
[att~=val]
Match when the element's "att" attribute value is a
space-separated list of "words", one of which is exactly "val".
If this selector is used, the words in the value must not
contain spaces (since they are separated by spaces).
[att|=val]
Match when the element's "att" attribute value is a
hyphen-separated list of "words", beginning with "val". The
match always starts at the beginning of the attribute value.
This is primarily intended to allow language subcode matches
(e.g., the "lang" attribute in HTML) as described in RFC 3066
([RFC3066]).
Attribute values must be identifiers or strings. The case-sensitivity
of attribute names and values in selectors depends on the document
language.
Example(s):
For example, the following attribute selector matches all H1 elements
that specify the "title" attribute, whatever its value:
h1[title] { color: blue; }
Example(s):
In the following example, the selector matches all SPAN elements whose
"class" attribute has exactly the value "example":
span[class=example] { color: blue; }
Multiple attribute selectors can be used to refer to several
attributes of an element, or even several times to the same attribute.
Example(s):
Here, the selector matches all SPAN elements whose "hello" attribute
has exactly the value "Cleveland" and whose "goodbye" attribute has
exactly the value "Columbus":
span[hello="Cleveland"][goodbye="Columbus"] { color: blue; }
Example(s):
The following selectors illustrate the differences between "=" and
"~=". The first selector will match, for example, the value "copyright
copyleft copyeditor" for the "rel" attribute. The second selector will
only match when the "href" attribute has the value
"http://www.w3.org/".
a[rel~="copyright"]
a[href="http://www.w3.org/"]
Example(s):
The following rule hides all elements for which the value of the
"lang" attribute is "fr" (i.e., the language is French).
*[lang=fr] { display : none }
Example(s):
The following rule will match for values of the "lang" attribute that
begin with "en", including "en", "en-US", and "en-cockney":
*[lang|="en"] { color : red }
Example(s):
Similarly, the following aural style sheet rules allow a script to be
read aloud in different voices for each role:
DIALOGUE[character=romeo]
{ voice-family: "Laurence Olivier", charles, male }
DIALOGUE[character=juliet]
{ voice-family: "Vivien Leigh", victoria, female }
5.8.2 Default attribute values in DTDs
Matching takes place on attribute values in the document tree. Default
attribute values may be defined in a DTD or elsewhere, but cannot
always be selected by attribute selectors. Style sheets should be
designed so that they work even if the default values are not included
in the document tree.
More precisely, a UA is not required to read an "external subset" of
the DTD but is required to look for default attribute values in the
document's "internal subset." (See [XML10] for definitions of these
subsets.)
A UA that recognizes an XML namespace [XMLNAMESPACES] is not required
to use its knowledge of that namespace to treat default attribute
values as if they were present in the document. (E.g., an XHTML UA is
not required to use its built-in knowledge of the XHTML DTD.)
Note that, typically, implementations choose to ignore external
subsets.
Example(s):
Example:
For example, consider an element EXAMPLE with an attribute "notation"
that has a default value of "decimal". The DTD fragment might be
If the style sheet contains the rules
EXAMPLE[notation=decimal] { /*... default property settings ...*/ }
EXAMPLE[notation=octal] { /*... other settings...*/ }
the first rule will not match elements whose "notation" attribute is
set by default, i.e. not set explicitly. To catch all cases, the
attribute selector for the default value must be dropped:
EXAMPLE { /*... default property settings ...*/ }
EXAMPLE[notation=octal] { /*... other settings...*/ }
Here, because the selector EXAMPLE[notation=octal] is more specific
than the tag selector alone, the style declarations in the second rule
will override those in the first for elements that have a "notation"
attribute value of "octal". Care has to be taken that all property
declarations that are to apply only to the default case are overridden
in the non-default cases' style rules.
5.8.3 Class selectors
Working with HTML, authors may use the period (.) notation as an
alternative to the ~= notation when representing the class attribute.
Thus, for HTML, div.value and div[class~=value] have the same meaning.
The attribute value must immediately follow the "period" (.). UAs may
apply selectors using the period (.) notation in XML documents if the
UA has namespace specific knowledge that allows it to determine which
attribute is the "class" attribute for the respective namespace. One
such example of namespace specific knowledge is the prose in the
specification for a particular namespace (e.g. SVG 1.1 [SVG11]
describes the SVG "class" attribute and how a UA should interpret it,
and similarly MathML 2.0 [MATH20] describes the MathML "class"
attribute.)
Example(s):
For example, we can assign style information to all elements with
class~="pastoral" as follows:
*.pastoral { color: green } /* all elements with class~=pastoral */
or just
.pastoral { color: green } /* all elements with class~=pastoral */
The following assigns style only to H1 elements with
class~="pastoral":
H1.pastoral { color: green } /* H1 elements with class~=pastoral */
Given these rules, the first H1 instance below would not have green
text, while the second would:

Not green

Very green

To match a subset of "class" values, each value must be preceded by a
".".
Example(s):
For example, the following rule matches any P element whose "class"
attribute has been assigned a list of space-separated values that
includes "pastoral" and "marine":
p.marine.pastoral { color: green }
This rule matches when class="pastoral blue aqua marine" but does not
match for class="pastoral blue".
Note. CSS gives so much power to the "class" attribute, that authors
could conceivably design their own "document language" based on
elements with almost no associated presentation (such as DIV and SPAN
in HTML) and assigning style information through the "class"
attribute. Authors should avoid this practice since the structural
elements of a document language often have recognized and accepted
meanings and author-defined classes may not.
Note: If an element has multiple class attributes, their values must
be concatenated with spaces between the values before searching for
the class. As of this time the working group is not aware of any
manner in which this situation can be reached, however, so this
behavior is explicitly non-normative in this specification.
5.9 ID selectors
Document languages may contain attributes that are declared to be of
type ID. What makes attributes of type ID special is that no two such
attributes can have the same value; whatever the document language, an
ID attribute can be used to uniquely identify its element. In HTML all
ID attributes are named "id"; XML applications may name ID attributes
differently, but the same restriction applies.
The ID attribute of a document language allows authors to assign an
identifier to one element instance in the document tree. CSS ID
selectors match an element instance based on its identifier. A CSS ID
selector contains a "#" immediately followed by the ID value, which
must be an identifier.
Note that CSS does not specify how a UA knows the ID attribute of an
element. The UA may, e.g., read a document's DTD, have the information
hard-coded or ask the user.
Example(s):
The following ID selector matches the H1 element whose ID attribute
has the value "chapter1":
h1#chapter1 { text-align: center }
In the following example, the style rule matches the element that has
the ID value "z98y". The rule will thus match for the P element:
Match P

Wide text

In the next example, however, the style rule will only match an H1
element that has an ID value of "z98y". The rule will not match the P
element in this example:
Match H1 only

Wide text

ID selectors have a higher specificity than attribute selectors. For
example, in HTML, the selector #p123 is more specific than [id=p123]
in terms of the cascade.
Note. In XML 1.0 [XML10], the information about which attribute
contains an element's IDs is contained in a DTD. When parsing XML, UAs
do not always read the DTD, and thus may not know what the ID of an
element is. If a style sheet designer knows or suspects that this will
be the case, he should use normal attribute selectors instead:
[name=p371] instead of #p371. However, the cascading order of normal
attribute selectors is different from ID selectors. It may be
necessary to add an "!important" priority to the declarations:
[name=p371] {color: red ! important}.
If an element has multiple ID attributes, all of them must be treated
as IDs for that element for the purposes of the ID selector. Such a
situation could be reached using mixtures of xml:id [XMLID], DOM3 Core
[DOM-LEVEL-3-CORE], XML DTDs [XML10] and namespace-specific knowledge.
5.10 Pseudo-elements and pseudo-classes
In CSS 2.1, style is normally attached to an element based on its
position in the document tree. This simple model is sufficient for
many cases, but some common publishing scenarios may not be possible
due to the structure of the document tree. For instance, in HTML 4
(see [HTML4]), no element refers to the first line of a paragraph, and
therefore no simple CSS selector may refer to it.
CSS introduces the concepts of pseudo-elements and pseudo-classes to
permit formatting based on information that lies outside the document
tree.
* Pseudo-elements create abstractions about the document tree beyond
those specified by the document language. For instance, document
languages do not offer mechanisms to access the first letter or
first line of an element's content. CSS pseudo-elements allow
style sheet designers to refer to this otherwise inaccessible
information. Pseudo-elements may also provide style sheet
designers a way to assign style to content that does not exist in
the source document (e.g., the :before and :after pseudo-elements
give access to generated content).
* Pseudo-classes classify elements on characteristics other than
their name, attributes or content; in principle characteristics
that cannot be deduced from the document tree. Pseudo-classes may
be dynamic, in the sense that an element may acquire or lose a
pseudo-class while a user interacts with the document. The
exceptions are ':first-child', which can be deduced from the
document tree, and ':lang()', which can be deduced from the
document tree in some cases.
Neither pseudo-elements nor pseudo-classes appear in the document
source or document tree.
Pseudo-classes are allowed anywhere in selectors while pseudo-elements
may only be appended after the last simple selector of the selector.
Pseudo-element and pseudo-class names are case-insensitive.
Some pseudo-classes are mutually exclusive, while others can be
applied simultaneously to the same element. In case of conflicting
rules, the normal cascading order determines the outcome.
5.11 Pseudo-classes
5.11.1 :first-child pseudo-class
The :first-child pseudo-class matches an element that is the first
child element of some other element.
Example(s):
In the following example, the selector matches any P element that is
the first child of a DIV element. The rule suppresses indentation for
the first paragraph of a DIV:
div > p:first-child { text-indent: 0 }
This selector would match the P inside the DIV of the following
fragment:

The last P before the note.

The first P inside the note.

but would not match the second P in the following fragment:

The last P before the note.

Note

The first P inside the note.

Example(s):
The following rule sets the font weight to 'bold' for any EM element
that is some descendant of a P element that is a first child:
p:first-child em { font-weight : bold }
Note that since anonymous boxes are not part of the document tree,
they are not counted when calculating the first child.
For example, the EM in:

abc default
is the first child of the P.
The following two selectors are equivalent:
* > a:first-child /* A is first child of any element */
a:first-child /* Same */
5.11.2 The link pseudo-classes: :link and :visited
User agents commonly display unvisited links differently from
previously visited ones. CSS provides the pseudo-classes ':link' and
':visited' to distinguish them:
* The :link pseudo-class applies for links that have not yet been
visited.
* The :visited pseudo-class applies once the link has been visited
by the user.
UAs may return a visited link to the (unvisited) ':link' state at some
point.
The two states are mutually exclusive.
The document language determines which elements are hyperlink source
anchors. For example, in HTML4, the link pseudo-classes apply to A
elements with an "href" attribute. Thus, the following two CSS 2.1
declarations have similar effect:
a:link { color: red }
:link { color: red }
Example(s):
If the following link:
external link
has been visited, this rule:
a.external:visited { color: blue }
will cause it to be blue.
Note. It is possible for style sheet authors to abuse the :link and
:visited pseudo-classes to determine which sites a user has visited
without the user's consent.
UAs may therefore treat all links as unvisited links, or implement
other measures to preserve the user's privacy while rendering visited
and unvisited links differently. See [P3P] for more information about
handling privacy.
5.11.3 The dynamic pseudo-classes: :hover, :active, and :focus
Interactive user agents sometimes change the rendering in response to
user actions. CSS provides three pseudo-classes for common cases:
* The :hover pseudo-class applies while the user designates an
element (with some pointing device), but does not activate it. For
example, a visual user agent could apply this pseudo-class when
the cursor (mouse pointer) hovers over a box generated by the
element. User agents not supporting interactive media do not have
to support this pseudo-class. Some conforming user agents
supporting interactive media may not be able to support this
pseudo-class (e.g., a pen device).
* The :active pseudo-class applies while an element is being
activated by the user. For example, between the times the user
presses the mouse button and releases it.
* The :focus pseudo-class applies while an element has the focus
(accepts keyboard events or other forms of text input).
An element may match several pseudo-classes at the same time.
CSS doesn't define which elements may be in the above states, or how
the states are entered and left. Scripting may change whether elements
react to user events or not, and different devices and UAs may have
different ways of pointing to, or activating elements.
CSS 2.1 doesn't define if the parent of an element that is ':active'
or ':hover' is also in that state.
User agents are not required to reflow a currently displayed document
due to pseudo-class transitions. For instance, a style sheet may
specify that the 'font-size' of an :active link should be larger than
that of an inactive link, but since this may cause letters to change
position when the reader selects the link, a UA may ignore the
corresponding style rule.
Example(s):
a:link { color: red } /* unvisited links */
a:visited { color: blue } /* visited links */
a:hover { color: yellow } /* user hovers */
a:active { color: lime } /* active links */
Note that the A:hover must be placed after the A:link and A:visited
rules, since otherwise the cascading rules will hide the 'color'
property of the A:hover rule. Similarly, because A:active is placed
after A:hover, the active color (lime) will apply when the user both
activates and hovers over the A element.
Example(s):
An example of combining dynamic pseudo-classes:
a:focus { background: yellow }
a:focus:hover { background: white }
The last selector matches A elements that are in pseudo-class :focus
and in pseudo-class :hover.
For information about the presentation of focus outlines, please
consult the section on dynamic focus outlines.
Note. In CSS1, the ':active' pseudo-class was mutually exclusive with
':link' and ':visited'. That is no longer the case. An element can be
both ':visited' and ':active' (or ':link' and ':active') and the
normal cascading rules determine which style declarations apply.
Note. Also note that in CSS1, the ':active' pseudo-class only applied
to links.
5.11.4 The language pseudo-class: :lang
If the document language specifies how the human language of an
element is determined, it is possible to write selectors in CSS that
match an element based on its language. For example, in HTML [HTML4],
the language is determined by a combination of the "lang" attribute,
the META element, and possibly by information from the protocol (such
as HTTP headers). XML uses an attribute called xml:lang, and there may
be other document language-specific methods for determining the
language.
The pseudo-class ':lang(C)' matches if the element is in language C.
Whether there is a match is based solely on the identifier C being
either equal to, or a hyphen-separated substring of, the element's
language value, in the same way as if performed by the '|=' operator.
The identifier C doesn't have to be a valid language name.
C must not be empty.
Note: It is recommended that documents and protocols indicate language
using codes from RFC 3066 [RFC3066] or its successor, and by means of
"xml:lang" attributes in the case of XML-based documents [XML10]. See
"FAQ: Two-letter or three-letter language codes."
Example(s):
The following rules set the quotation marks for an HTML document that
is either in Canadian French or German:
html:lang(fr-ca) { quotes: '« ' ' »' }
html:lang(de) { quotes: '»' '«' '\2039' '\203A' }
:lang(fr) > Q { quotes: '« ' ' »' }
:lang(de) > Q { quotes: '»' '«' '\2039' '\203A' }
The second pair of rules actually set the 'quotes' property on Q
elements according to the language of its parent. This is done because
the choice of quote marks is typically based on the language of the
element around the quote, not the quote itself: like this piece of
French "à l'improviste" in the middle of an English text uses the
English quotation marks.
Note the difference between [lang|=xx] and :lang(xx). In this HTML
example, only the BODY matches [lang|=fr] (because it has a LANG
attribute) but both the BODY and the P match :lang(fr) (because both
are in French).

Je suis Français.

5.12 Pseudo-elements
5.12.1 The :first-line pseudo-element
The :first-line pseudo-element applies special styles to the contents
of the first formatted line of a paragraph. For instance:
p:first-line { text-transform: uppercase }
The above rule means "change the letters of the first line of every
paragraph to uppercase". However, the selector "P:first-line" does not
match any real HTML element. It does match a pseudo-element that
conforming user agents will insert at the beginning of every
paragraph.
Note that the length of the first line depends on a number of factors,
including the width of the page, the font size, etc. Thus, an ordinary
HTML paragraph such as:

This is a somewhat long HTML
paragraph that will be broken into several
lines. The first line will be identified
by a fictional tag sequence. The other lines
will be treated as ordinary lines in the
paragraph.

the lines of which happen to be broken as follows:
THIS IS A SOMEWHAT LONG HTML PARAGRAPH THAT
will be broken into several lines. The first
line will be identified by a fictional tag
sequence. The other lines will be treated as
ordinary lines in the paragraph.
might be "rewritten" by user agents to include the fictional tag
sequence for :first-line. This fictional tag sequence helps to show
how properties are inherited.

This is a somewhat long HTML
paragraph that will be broken into several
lines. The first line will be identified
by a fictional tag sequence. The other lines
will be treated as ordinary lines in the
paragraph.

If a pseudo-element breaks up a real element, the desired effect can
often be described by a fictional tag sequence that closes and then
re-opens the element. Thus, if we mark up the previous paragraph with
a SPAN element:

This is a somewhat long HTML
paragraph that will be broken into several
lines. The first line will be identified
by a fictional tag sequence. The other lines
will be treated as ordinary lines in the
paragraph.

the user agent could simulate start and end tags for SPAN when
inserting the fictional tag sequence for :first-line.

This is a
somewhat long HTML
paragraph that will be
broken into several
lines. The first line will be identified
by a fictional tag sequence. The other lines
will be treated as ordinary lines in the
paragraph.

The :first-line pseudo-element can only be attached to a block-level
element, inline-block, table-caption or a table-cell.
The "first formatted line" of an element may occur inside a
block-level descendant in the same flow (i.e., a block-level
descendant that is not positioned and not a float). E.g., the first
line of the DIV in

This line...

is the first line of
the P (assuming that both P and DIV are block-level).
The first line of a table-cell or inline-block cannot be the first
formatted line of an ancestor element. Thus, in

HelloGoodbye

etcetera

the
first formatted line of the DIV is not the line "Hello".
Note that the first line of the P in this fragment:

First...
doesn't contain any letters (assuming the default style for BR in HTML
4). The word "First" is not on the first formatted line.
A UA should act as if the fictional start tags of the first-line
pseudo-elements were nested just inside the innermost enclosing
block-level element. (Since CSS1 and CSS2 were silent on this case,
authors should not rely on this behavior.) Here is an example. The
fictional tag sequence for

First paragraph

Second paragraph

is

First paragraph

Second paragraph

The :first-line pseudo-element is similar to an inline-level element,
but with certain restrictions. The following properties apply to a
:first-line pseudo-element: font properties, color property,
background properties, 'word-spacing', 'letter-spacing',
'text-decoration', 'vertical-align', 'text-transform', 'line-height',.
UAs may apply other properties as well.
5.12.2 The :first-letter pseudo-element
The :first-letter pseudo-element must select the first letter of the
first line of a block, if it is not preceded by any other content
(such as images or inline tables) on its line. The :first-letter
pseudo-element may be used for "initial caps" and "drop caps", which
are common typographical effects. This type of initial letter is
similar to an inline-level element if its 'float' property is 'none',
otherwise it is similar to a floated element.
These are the properties that apply to :first-letter pseudo-elements:
font properties, 'text-decoration', 'text-transform',
'letter-spacing', 'word-spacing' (when appropriate), 'line-height',
'float', 'vertical-align' (only if 'float' is 'none'), margin
properties, padding properties, border properties, color property,
background properties. UAs may apply other properties as well. To
allow UAs to render a typographically correct drop cap or initial cap,
the UA may choose a line-height, width and height based on the shape
of the letter, unlike for normal elements. CSS3 is expected to have
specific properties that apply to first-letter.
This example shows a possible rendering of an initial cap. Note that
the 'line-height' that is inherited by the first-letter pseudo-element
is 1.1, but the UA in this example has computed the height of the
first letter differently, so that it doesn't cause any unnecessary
space between the first two lines. Also note that the fictional start
tag of the first letter is inside the SPAN, and thus the font weight
of the first letter is normal, not bold as the SPAN:
p { line-height: 1.1 }
p:first-letter { font-size: 3em; font-weight: normal }
span { font-weight: bold }
...

This example might be formatted as follows:
Image illustrating the combined effect of the :first-letter and
:first-line pseudo-elements [D]
The fictional tag sequence is:

T
he first
few words of an article in the Economist.

Note that the :first-letter pseudo-element tags abut the content
(i.e., the initial character), while the :first-line pseudo-element
start tag is inserted right after the start tag of the block element.
In order to achieve traditional drop caps formatting, user agents may
approximate font sizes, for example to align baselines. Also, the
glyph outline may be taken into account when formatting.
Punctuation (i.e, characters defined in Unicode [UNICODE] in the
"open" (Ps), "close" (Pe), "initial" (Pi). "final" (Pf) and "other"
(Po) punctuation classes), that precedes or follows the first letter
should be included, as in:
Quotes that precede the first letter should be included. [D]
The ':first-letter' also applies if the first letter is in fact a
digit, e.g., the "6" in "67 million dollars is a lot of money."
The :first-letter pseudo-element applies to block, list-item,
table-cell, table-caption and inline-block elements.
The :first-letter pseudo-element can be used with all such elements
that contain text, or that have a descendant in the same flow that
contains text. A UA should act as if the fictional start tag of the
first-letter pseudo-element is just before the first text of the
element, even if that first text is in a descendant.
Example(s):
Here is an example. The fictional tag sequence for this HTML fragment:

The first text.
is:

The first text.
The first letter of a table-cell or inline-block cannot be the first
letter of an ancestor element. Thus, in

HelloGoodbye

etcetera

the first letter of
the DIV is not the letter "H". In fact, the DIV doesn't have a first
letter.
The first letter must occur on the first formatted line. For example,
in this fragment:

First... the first line doesn't contain any
letters and ':first-letter' doesn't match anything (assuming the
default style for BR in HTML 4). In particular, it does not match the
"F" of "First."
If an element is a list item ('display: list-item'), the
':first-letter' applies to the first letter in the principal box after
the marker. UAs may ignore ':first-letter' on list items with
'list-style-position: inside'. If an element has ':before' or ':after'
content, the ':first-letter applies to the first letter of the element
including that content.
E.g., after the rule 'p:before {content: "Note: "}', the selector
'p:first-letter' matches the "N" of "Note".
Some languages may have specific rules about how to treat certain
letter combinations. In Dutch, for example, if the letter combination
"ij" appears at the beginning of a word, both letters should be
considered within the :first-letter pseudo-element.
If the letters that would form the first-letter are not in the same
element, such as "'T" in

'T..., the UA may create a
first-letter pseudo-element from one of the elements, both elements,
or simply not create a pseudo-element.
Similarly, if the first letter(s) of the block are not at the start of
the line (for example due to bidirectional reordering), then the UA
need not create the pseudo-element(s).
Example(s):
The following example illustrates how overlapping pseudo-elements may
interact. The first letter of each P element will be green with a font
size of '24pt'. The rest of the first formatted line will be 'blue'
while the rest of the paragraph will be 'red'.
p { color: red; font-size: 12pt }
p:first-letter { color: green; font-size: 200% }
p:first-line { color: blue }

Some text that ends up on two lines

Assuming that a line break will occur before the word "ends", the
fictional tag sequence for this fragment might be:

S
ome text that
ends up on two lines

Note that the :first-letter element is inside the :first-line element.
Properties set on :first-line are inherited by :first-letter, but are
overridden if the same property is set on :first-letter.
5.12.3 The :before and :after pseudo-elements
The ':before' and ':after' pseudo-elements can be used to insert
generated content before or after an element's content. They are
explained in the section on generated text.
Example(s):
h1:before {content: counter(chapno, upper-roman) ". "}
When the :first-letter and :first-line pseudo-elements are combined
with :before and :after, they apply to the first letter or line of the
element including the inserted text.
Example(s):
p.special:before {content: "Special! "}
p.special:first-letter {color: #ffd800}
This will render the "S" of "Special!" in gold.
_________________________________________________________________
_________________________________________________________________
6 Assigning property values, Cascading, and Inheritance
Contents
* 6.1 Specified, computed, and actual values
+ 6.1.1 Specified values
+ 6.1.2 Computed values
+ 6.1.3 Used values
+ 6.1.4 Actual values
* 6.2 Inheritance
+ 6.2.1 The 'inherit' value
* 6.3 The @import rule
* 6.4 The cascade
+ 6.4.1 Cascading order
+ 6.4.2 !important rules
+ 6.4.3 Calculating a selector's specificity
+ 6.4.4 Precedence of non-CSS presentational hints
6.1 Specified, computed, and actual values
Once a user agent has parsed a document and constructed a document
tree, it must assign, for every element in the tree, a value to every
property that applies to the target media type.
The final value of a property is the result of a four-step
calculation: the value is determined through specification (the
"specified value"), then resolved into a value that is used for
inheritance (the "computed value"), then converted into an absolute
value if necessary (the "used value"), and finally transformed
according to the limitations of the local environment (the "actual
value").
6.1.1 Specified values
User agents must first assign a specified value to each property based
on the following mechanisms (in order of precedence):
1. If the cascade results in a value, use it.
2. Otherwise, if the property is inherited and the element is not the
root of the document tree, use the computed value of the parent
element.
3. Otherwise use the property's initial value. The initial value of
each property is indicated in the property's definition.
6.1.2 Computed values
Specified values are resolved to computed values during the cascade;
for example URIs are made absolute and 'em' and 'ex' units are
computed to pixel or absolute lengths. Computing a value never
requires the user agent to render the document.
The computed value of URIs that the UA cannot resolve to absolute URIs
is the specified value.
When the specified value is not 'inherit', the computed value of a
property is determined as specified by the Computed Value line in the
definition of the property. See the section on inheritance for the
definition of computed values when the specified value is 'inherit'.
The computed value exists even when the property doesn't apply, as
defined by the 'Applies To' line. However, some properties may define
the computed value of a property for an element to depend on whether
the property applies to that element.
6.1.3 Used values
Computed values are processed as far as possible without formatting
the document. Some values, however, can only be determined when the
document is being laid out. For example, if the width of an element is
set to be a certain percentage of its containing block, the width
cannot be determined until the width of the containing block has been
determined. The used value is the result of taking the computed value
and resolving any remaining dependencies into an absolute value.
6.1.4 Actual values
A used value is in principle the value used for rendering, but a user
agent may not be able to make use of the value in a given environment.
For example, a user agent may only be able to render borders with
integer pixel widths and may therefore have to approximate the
computed width, or the user agent may be forced to use only black and
white shades instead of full colour. The actual value is the used
value after any approximations have been applied.
6.2 Inheritance
Some values are inherited by the children of an element in the
document tree, as described above. Each property defines whether it is
inherited or not.
Suppose there is an H1 element with an emphasizing element (EM)
inside:

The headline is important!

If no color has been assigned to the EM element, the emphasized "is"
will inherit the color of the parent element, so if H1 has the color
blue, the EM element will likewise be in blue.
When inheritance occurs, elements inherit computed values. The
computed value from the parent element becomes both the specified
value and the computed value on the child.
Example(s):
For example, given the following style sheet:
body { font-size: 10pt }
h1 { font-size: 130% }
and this document fragment:

A large heading

the 'font-size' property for the H1 element will have the computed
value '13pt' (130% times 10pt, the parent's value). Since the computed
value of 'font-size' is inherited, the EM element will have the
computed value '13pt' as well. If the user agent does not have the
13pt font available, the actual value of 'font-size' for both H1 and
EM might be, for example, '12pt'.
6.2.1 The 'inherit' value
Each property may also have a specified value of 'inherit', which
means that, for a given element, the property takes the same computed
value as the property for the element's parent. The 'inherit' value
can be used to strengthen inherited values, and it can also be used on
properties that are not normally inherited.
If the 'inherit' value is set on the root element, the property is
assigned its initial value.
Example(s):
In the example below, the 'color' and 'background' properties are set
on the BODY element. On all other elements, the 'color' value will be
inherited and the background will be transparent. If these rules are
part of the user's style sheet, black text on a white background will
be enforced throughout the document.
body {
color: black !important;
background: white !important;
}
* {
color: inherit !important;
background: transparent !important;
}
6.3 The @import rule
The '@import' rule allows users to import style rules from other style
sheets. Any @import rules must precede all other rules (except the
@charset rule, if present). The '@import' keyword must be followed by
the URI of the style sheet to include. A string is also allowed; it
will be interpreted as if it had url(...) around it.
Example(s):
The following lines are equivalent in meaning and illustrate both
'@import' syntaxes (one with "url()" and one with a bare string):
@import "mystyle.css";
@import url("mystyle.css");
So that user agents can avoid retrieving resources for unsupported
media types, authors may specify media-dependent @import rules. These
conditional imports specify comma-separated media types after the URI.
Example(s):
The following rules illustrate how @import rules can be made
media-dependent:
@import url("fineprint.css") print;
@import url("bluish.css") projection, tv;
In the absence of any media types, the import is unconditional.
Specifying 'all' for the medium has the same effect.
6.4 The cascade
Style sheets may have three different origins: author, user, and user
agent.
* Author. The author specifies style sheets for a source document
according to the conventions of the document language. For
instance, in HTML, style sheets may be included in the document or
linked externally.
* User: The user may be able to specify style information for a
particular document. For example, the user may specify a file that
contains a style sheet or the user agent may provide an interface
that generates a user style sheet (or behaves as if it did).
* User agent: Conforming user agents must apply a default style
sheet (or behave as if they did). A user agent's default style
sheet should present the elements of the document language in ways
that satisfy general presentation expectations for the document
language (e.g., for visual browsers, the EM element in HTML is
presented using an italic font). See A sample style sheet for HTML
for a recommended default style sheet for HTML documents.
Note that the user may modify system settings (e.g. system colors)
that affect the default style sheet. However, some user agent
implementations make it impossible to change the values in the
default style sheet.
Style sheets from these three origins will overlap in scope, and they
interact according to the cascade.
The CSS cascade assigns a weight to each style rule. When several
rules apply, the one with the greatest weight takes precedence.
By default, rules in author style sheets have more weight than rules
in user style sheets. Precedence is reversed, however, for
"!important" rules. All user and author rules have more weight than
rules in the UA's default style sheet.
6.4.1 Cascading order
To find the value for an element/property combination, user agents
must apply the following sorting order:
1. Find all declarations that apply to the element and property in
question, for the target media type. Declarations apply if the
associated selector matches the element in question.
2. Sort according to importance (normal or important) and origin
(author, user, or user agent). In ascending order of precendence:
1. user agent declarations
2. user normal declarations
3. author normal declarations
4. author important declarations
5. user important declarations
3. Sort rules with the same importance and origin by specificity of
selector: more specific selectors will override more general ones.
Pseudo-elements and pseudo-classes are counted as normal elements
and classes, respectively.
4. Finally, sort by order specified: if two declarations have the
same weight, origin and specificity, the latter specified wins.
Declarations in imported style sheets are considered to be before
any declarations in the style sheet itself.
Apart from the "!important" setting on individual declarations, this
strategy gives author's style sheets higher weight than those of the
reader. User agents must give the user the ability to turn off the
influence of specific author style sheets, e.g., through a pull-down
menu. Conformance to UAAG 1.0 checkpoint 4.14 satisfies this condition
[UAAG10].
6.4.2 !important rules
CSS attempts to create a balance of power between author and user
style sheets. By default, rules in an author's style sheet override
those in a user's style sheet (see cascade rule 3).
However, for balance, an "!important" declaration (the delimiter token
"!" and keyword "important" follow the declaration) takes precedence
over a normal declaration. Both author and user style sheets may
contain "!important" declarations, and user "!important" rules
override author "!important" rules. This CSS feature improves
accessibility of documents by giving users with special requirements
(large fonts, color combinations, etc.) control over presentation.
Declaring a shorthand property (e.g., 'background') to be "!important"
is equivalent to declaring all of its sub-properties to be
"!important".
Example(s):
The first rule in the user's style sheet in the following example
contains an "!important" declaration, which overrides the
corresponding declaration in the author's style sheet. The second
declaration will also win due to being marked "!important". However,
the third rule in the user's style sheet is not "!important" and will
therefore lose to the second rule in the author's style sheet (which
happens to set style on a shorthand property). Also, the third author
rule will lose to the second author rule since the second rule is
"!important". This shows that "!important" declarations have a
function also within author style sheets.
/* From the user's style sheet */
p { text-indent: 1em ! important }
p { font-style: italic ! important }
p { font-size: 18pt }
/* From the author's style sheet */
p { text-indent: 1.5em !important }
p { font: normal 12pt sans-serif !important }
p { font-size: 24pt }
6.4.3 Calculating a selector's specificity
A selector's specificity is calculated as follows:
* count 1 if the declaration is from is a 'style' attribute rather
than a rule with a selector, 0 otherwise (= a) (In HTML, values of
an element's "style" attribute are style sheet rules. These rules
have no selectors, so a=1, b=0, c=0, and d=0.)
* count the number of ID attributes in the selector (= b)
* count the number of other attributes and pseudo-classes in the
selector (= c)
* count the number of element names and pseudo-elements in the
selector (= d)
The specificity is based only on the form of the selector. In
particular, a selector of the form "[id=p33]" is counted as an
attribute selector (a=0, b=0, c=1, d=0), even if the id attribute is
defined as an "ID" in the source document's DTD.
Concatenating the four numbers a-b-c-d (in a number system with a
large base) gives the specificity.
Example(s):
Some examples:
* {} /* a=0 b=0 c=0 d=0 -> specificity = 0,0,0,0 */
li {} /* a=0 b=0 c=0 d=1 -> specificity = 0,0,0,1 */
li:first-line {} /* a=0 b=0 c=0 d=2 -> specificity = 0,0,0,2 */
ul li {} /* a=0 b=0 c=0 d=2 -> specificity = 0,0,0,2 */
ul ol+li {} /* a=0 b=0 c=0 d=3 -> specificity = 0,0,0,3 */
h1 + *[rel=up]{} /* a=0 b=0 c=1 d=1 -> specificity = 0,0,1,1 */
ul ol li.red {} /* a=0 b=0 c=1 d=3 -> specificity = 0,0,1,3 */
li.red.level {} /* a=0 b=0 c=2 d=1 -> specificity = 0,0,2,1 */
#x34y {} /* a=0 b=1 c=0 d=0 -> specificity = 0,1,0,0 */
style="" /* a=1 b=0 c=0 d=0 -> specificity = 1,0,0,0 */

In the above example, the color of the P element would be green. The
declaration in the "style" attribute will override the one in the
STYLE element because of cascading rule 3, since it has a higher
specificity.
6.4.4 Precedence of non-CSS presentational hints
The UA may choose to honor presentational attributes in an HTML source
document. If so, these attributes are translated to the corresponding
CSS rules with specificity equal to 0, and are treated as if they were
inserted at the start of the author style sheet. They may therefore be
overridden by subsequent style sheet rules. In a transition phase,
this policy will make it easier for stylistic attributes to coexist
with style sheets.
For HTML, any attribute that is not in the following list should be
considered presentational: abbr, accept-charset, accept, accesskey,
action, alt, archive, axis, charset, checked, cite, class, classid,
code, codebase, codetype, colspan, coords, data, datetime, declare,
defer, dir, disabled, enctype, for, headers, href, hreflang,
http-equiv, id, ismap, label, lang, language, longdesc, maxlength,
media, method, multiple, name, nohref, object, onblur, onchange,
onclick, ondblclick, onfocus, onkeydown, onkeypress, onkeyup, onload,
onload, onmousedown, onmousemove, onmouseout, onmouseover, onmouseup,
onreset, onselect, onsubmit, onunload, onunload, profile, prompt,
readonly, rel, rev, rowspan, scheme, scope, selected, shape, span,
src, standby, start, style, summary, title, type (except on LI, OL and
UL elements), usemap, value, valuetype, version.
For other languages, all document language-based styling should be
handled in the user agent style sheet.
Example(s):
The following user style sheet would override the font weight of 'b'
elements in all documents, and the color of 'font' elements with color
attributes in XML documents. It would not affect the color of any
'font' elements with color attributes in HTML documents:
b { font-weight: normal; }
font[color] { color: orange; }
The following, however, would override the color of font elements in
all documents:
font[color] { color: orange ! important; }
_________________________________________________________________
_________________________________________________________________
7 Media types
Contents
* 7.1 Introduction to media types
* 7.2 Specifying media-dependent style sheets
+ 7.2.1 The @media rule
* 7.3 Recognized media types
+ 7.3.1 Media groups
7.1 Introduction to media types
One of the most important features of style sheets is that they
specify how a document is to be presented on different media: on the
screen, on paper, with a speech synthesizer, with a braille device,
etc.
Certain CSS properties are only designed for certain media (e.g., the
'page-break-before' property only applies to paged media). On
occasion, however, style sheets for different media types may share a
property, but require different values for that property. For example,
the 'font-size' property is useful both for screen and print media.
The two media types are different enough to require different values
for the common property; a document will typically need a larger font
on a computer screen than on paper. Therefore, it is necessary to
express that a style sheet, or a section of a style sheet, applies to
certain media types.
7.2 Specifying media-dependent style sheets
There are currently two ways to specify media dependencies for style
sheets:
* Specify the target medium from a style sheet with the @media or
@import at-rules.
Example(s):
@import url("fancyfonts.css") screen;
@media print {
/* style sheet for print goes here */
}
* Specify the target medium within the document language. For
example, in HTML 4 ([HTML4]), the "media" attribute on the LINK
element specifies the target media of an external style sheet:

Link to a target medium

The body...
The @import rule is defined in the chapter on the cascade.
7.2.1 The @media rule
An @media rule specifies the target media types (separated by commas)
of a set of rules (delimited by curly braces). The @media construct
allows style sheet rules for various media in the same style sheet:
@media print {
body { font-size: 10pt }
}
@media screen {
body { font-size: 13px }
}
@media screen, print {
body { line-height: 1.2 }
}
Style rules outside of @media rules apply to all media types that the
style sheet applies to.
7.3 Recognized media types
The names chosen for CSS media types reflect target devices for which
the relevant properties make sense. In the following list of CSS media
types the names of media types are normative, but the descriptions are
informative. Likewise, the "Media" field in the description of each
property is informative.
all
Suitable for all devices.
braille
Intended for braille tactile feedback devices.
embossed
Intended for paged braille printers.
handheld
Intended for handheld devices (typically small screen, limited
bandwidth).
print
Intended for paged material and for documents viewed on screen
in print preview mode. Please consult the section on paged
media for information about formatting issues that are specific
to paged media.
projection
Intended for projected presentations, for example projectors.
Please consult the section on paged media for information about
formatting issues that are specific to paged media.
screen
Intended primarily for color computer screens.
speech
Intended for speech synthesizers. Note: CSS2 had a similar
media type called 'aural' for this purpose. See the appendix on
aural style sheets for details.
tty
Intended for media using a fixed-pitch character grid (such as
teletypes, terminals, or portable devices with limited display
capabilities). Authors should not use pixel units with the
"tty" media type.
tv
Intended for television-type devices (low resolution, color,
limited-scrollability screens, sound available).
Media type names are case-insensitive.
Media types are mutually exclusive in the sense that a user agent can
only support one media type when rendering a document. However, user
agents may use different media types on different canvases. For
example, a document may (simultaneously) be shown in 'screen' mode on
one canvas and 'print' mode on another canvas.
Note that a multimodal media type is still only one media type. The
'tv' media type, for example, is a multimodal media type that renders
both visually and aurally to a single canvas.
@media and @import rules with unknown media types are treated as if
the unknown media types are not present.
Example(s):
For example, in the following snippet, the rule on the P element
applies in 'screen' mode (even though the '3D' media type is not
known).
@media screen, 3D {
P { color: green; }
}
Note. Future updates of CSS may extend the list of media types.
Authors should not rely on media type names that are not yet defined
by a CSS specification.
7.3.1 Media groups
This section is informative, not normative.
Each CSS property definition specifies which media types the property
applies to. Since properties generally apply to several media types,
the "Applies to media" section of each property definition lists media
groups rather than individual media types. Each property applies to
all media types in the media groups listed in its definition.
CSS 2.1 defines the following media groups:
* continuous or paged.
* visual, audio, speech, or tactile.
* grid (for character grid devices), or bitmap.
* interactive (for devices that allow user interaction), or static
(for those that don't).
* all (includes all media types)
The following table shows the relationships between media groups and
media types:
CAPTION: Relationship between media groups and media types
Media Types Media Groups
continuous/paged visual/audio/speech/tactile grid/bitmap
interactive/static
braille continuous tactile grid both
embossed paged tactile grid static
handheld both visual, audio, speech both both
print paged visual bitmap static
projection paged visual bitmap interactive
screen continuous visual, audio bitmap both
speech continuous speech N/A both
tty continuous visual grid both
tv both visual, audio bitmap both
_________________________________________________________________
_________________________________________________________________
8 Box model
Contents
* 8.1 Box dimensions
* 8.2 Example of margins, padding, and borders
* 8.3 Margin properties: 'margin-top', 'margin-right',
'margin-bottom', 'margin-left', and 'margin'
+ 8.3.1 Collapsing margins
* 8.4 Padding properties: 'padding-top', 'padding-right',
'padding-bottom', 'padding-left', and 'padding'
* 8.5 Border properties
+ 8.5.1 Border width: 'border-top-width', 'border-right-width',
'border-bottom-width', 'border-left-width', and
'border-width'
+ 8.5.2 Border color: 'border-top-color', 'border-right-color',
'border-bottom-color', 'border-left-color', and
'border-color'
+ 8.5.3 Border style: 'border-top-style', 'border-right-style',
'border-bottom-style', 'border-left-style', and
'border-style'
+ 8.5.4 Border shorthand properties: 'border-top',
'border-right', 'border-bottom', 'border-left', and 'border'
* 8.6 The box model for inline elements in bidirection context
The CSS box model describes the rectangular boxes that are generated
for elements in the document tree and laid out according to the visual
formatting model.
8.1 Box dimensions
Each box has a content area (e.g., text, an image, etc.) and optional
surrounding padding, border, and margin areas; the size of each area
is specified by properties defined below. The following diagram shows
how these areas relate and the terminology used to refer to pieces of
margin, border, and padding:
Image illustrating the relationship between content, padding, borders,
and margins. [D]
The margin, border, and padding can be broken down into top, right,
bottom, and left segments (e.g., in the diagram, "LM" for left margin,
"RP" for right padding, "TB" for top border, etc.).
The perimeter of each of the four areas (content, padding, border, and
margin) is called an "edge", so each box has four edges:
content edge or inner edge
The content edge surrounds the rectangle given by the width and
height of the box, which often depend on the element's rendered
content. The four content edges define the box's content box.
padding edge
The padding edge surrounds the box padding. If the padding has
0 width, the padding edge is the same as the content edge. The
four padding edges define the box's padding box.
border edge
The border edge surrounds the box's border. If the border has 0
width, the border edge is the same as the padding edge. The
four border edges define the box's border box.
margin edge or outer edge
The margin edge surrounds the box margin. If the margin has 0
width, the margin edge is the same as the border edge. The four
margin edges define the box's margin box.
Each edge may be broken down into a top, right, bottom, and left edge.
The dimensions of the content area of a box -- the content width and
content height -- depend on several factors: whether the element
generating the box has the 'width' or 'height' property set, whether
the box contains text or other boxes, whether the box is a table, etc.
Box widths and heights are discussed in the chapter on visual
formatting model details.
The background style of the content, padding, and border areas of a
box is specified by the 'background' property of the generating
element. Margin backgrounds are always transparent.
8.2 Example of margins, padding, and borders
This example illustrates how margins, padding, and borders interact.
The example HTML document:

Examples of margins, padding, and borders

First element of list

Second element of list is
a bit longer to illustrate wrapping.

results in a document tree with (among other relationships) a UL
element that has two LI children.
The first of the following diagrams illustrates what this example
would produce. The second illustrates the relationship between the
margins, padding, and borders of the UL elements and those of its
children LI elements. (Image is not to scale.)
Image illustrating how parent and child margins, borders, and padding
relate. [D]
Note that:
* The content width for each LI box is calculated top-down; the
containing block for each LI box is established by the UL element.
* The margin box height of each LI box depends on its content
height, plus top and bottom padding, borders, and margins. Note
that vertical margins between the LI boxes collapse.
* The right padding of the LI boxes has been set to zero width (the
'padding' property). The effect is apparent in the second
illustration.
* The margins of the LI boxes are transparent -- margins are always
transparent -- so the background color (yellow) of the UL padding
and content areas shines through them.
* The second LI element specifies a dashed border (the
'border-style' property).
8.3 Margin properties: 'margin-top', 'margin-right', 'margin-bottom',
'margin-left', and 'margin'
Margin properties specify the width of the margin area of a box. The
'margin' shorthand property sets the margin for all four sides while
the other margin properties only set their respective side. These
properties apply to all elements, but vertical margins will not have
any effect on non-replaced inline elements.
The properties defined in this section refer to the
value type, which may take one of the following values:
Specifies a fixed width.
The percentage is calculated with respect to the width of the
generated box's containing block. Note that this is true for
'margin-top' and 'margin-bottom' as well. If the containing
block's width depends on this element, then the resulting
layout is undefined in CSS 2.1.
auto
See the section on calculating widths and margins for behavior.
Negative values for margin properties are allowed, but there may be
implementation-specific limits.
'margin-top', 'margin-bottom'
Value: | inherit
Initial: 0
Applies to: all elements except elements with table display types
other than table-caption, table and inline-table
Inherited: no
Percentages: refer to width of containing block
Media: visual
Computed value: the percentage as specified or the absolute length
These properties have no effect on non-replaced inline elements.
'margin-right', 'margin-left'
Value: | inherit
Initial: 0
Applies to: all elements except elements with table display types
other than table-caption, table and inline-table
Inherited: no
Percentages: refer to width of containing block
Media: visual
Computed value: the percentage as specified or the absolute length
These properties set the top, right, bottom, and left margin of a box.
Example(s):
h1 { margin-top: 2em }
'margin'
Value: {1,4} | inherit
Initial: see individual properties
Applies to: all elements except elements with table display types
other than table-caption, table and inline-table
Inherited: no
Percentages: refer to width of containing block
Media: visual
Computed value: see individual properties
The 'margin' property is a shorthand property for setting
'margin-top', 'margin-right', 'margin-bottom', and 'margin-left' at
the same place in the style sheet.
If there is only one value, it applies to all sides. If there are two
values, the top and bottom margins are set to the first value and the
right and left margins are set to the second. If there are three
values, the top is set to the first value, the left and right are set
to the second, and the bottom is set to the third. If there are four
values, they apply to the top, right, bottom, and left, respectively.
Example(s):
body { margin: 2em } /* all margins set to 2em */
body { margin: 1em 2em } /* top & bottom = 1em, right & left = 2em */
body { margin: 1em 2em 3em } /* top=1em, right=2em, bottom=3em, left=2em */
The last rule of the example above is equivalent to the example below:
body {
margin-top: 1em;
margin-right: 2em;
margin-bottom: 3em;
margin-left: 2em; /* copied from opposite side (right) */
}
8.3.1 Collapsing margins
In this specification, the expression collapsing margins means that
adjoining margins (no non-empty content, padding or border areas or
clearance separate them) of two or more boxes (which may be next to
one another or nested) combine to form a single margin.
In CSS 2.1, horizontal margins never collapse.
Vertical margins may collapse between certain boxes:
* Two or more adjoining vertical margins of block boxes in the
normal flow collapse. The resulting margin width is the maximum of
the adjoining margin widths. In the case of negative margins, the
maximum of the absolute values of the negative adjoining margins
is deducted from the maximum of the positive adjoining margins. If
there are no positive margins, the absolute maximum of the
negative adjoining margins is deducted from zero. Note. Adjoining
boxes may be generated by elements that are not related as
siblings or ancestors.
* Vertical margins between a floated box and any other box do not
collapse (not even between a float and its in-flow children).
* Vertical margins of elements with 'overflow' other than 'visible'
do not collapse with their in-flow children.
* Margins of absolutely positioned boxes do not collapse (not even
with their in-flow children).
* Margins of inline-block elements do not collapse (not even with
their in-flow children).
* If the top and bottom margins of a box are adjoining, then it is
possible for margins to collapse through it. In this case, the
position of the element depends on its relationship with the other
elements whose margins are being collapsed.
+ If the element's margins are collapsed with its parent's top
margin, the top border edge of the box is defined to be the
same as the parent's.
+ Otherwise, either the element's parent is not taking part in
the margin collapsing, or only the parent's bottom margin is
involved. The position of the element's top border edge is
the same as it would have been if the element had a non-zero
top border.
An element that has had clearance applied to it never collapses
its top margin with its parent block's bottom margin.
Note that the positions of elements that have been collapsed
through have no effect on the positions of the other elements with
whose margins they are being collapsed; the top border edge
position is only required for laying out descendants of these
elements.
* Margins of the root element's box do not collapse.
The bottom margin of an in-flow block-level element is always
adjoining to the top margin of its next in-flow block-level sibling,
unless that sibling has clearance.
The top margin of an in-flow block-level element is adjoining to its
first in-flow block-level child's top margin if the element has no top
border, no top padding, and the child has no clearance.
The bottom margin of an in-flow block-level element with a 'height' of
'auto' and 'min-height' less than the element's used height and
'max-height' greater than the element's used height is adjoining to
its last in-flow block-level child's bottom margin if the element has
no bottom padding or border.
An element's own margins are adjoining if the 'min-height' property is
zero, and it has neither top or bottom borders nor top or bottom
padding, and it has a 'height' of either 0 or 'auto', and it does not
contain a line box, and all of its in-flow children's margins (if any)
are adjoining.
When an element's own margins collapse, and that element has had
clearance applied to it, its top margin collapses with the adjoining
margins of subsequent siblings but that resulting margin does not
collapse with the bottom margin of the parent block.
Collapsing is based on the used value of 'padding', 'margin', and
'border' (i.e., after resolving any percentages). The collapsed margin
is calculated over the used value of the various margins.
Please consult the examples of margin, padding, and borders for an
illustration of collapsed margins.
8.4 Padding properties: 'padding-top', 'padding-right', 'padding-bottom',
'padding-left', and 'padding'
The padding properties specify the width of the padding area of a box.
The 'padding' shorthand property sets the padding for all four sides
while the other padding properties only set their respective side.
The properties defined in this section refer to the
value type, which may take one of the following values:
Specifies a fixed width.
The percentage is calculated with respect to the width of the
generated box's containing block, even for 'padding-top' and
'padding-bottom'. If the containing block's width depends on
this element, then the resulting layout is undefined in
CSS 2.1.
Unlike margin properties, values for padding values cannot be
negative. Like margin properties, percentage values for padding
properties refer to the width of the generated box's containing block.
'padding-top', 'padding-right', 'padding-bottom', 'padding-left'
Value: | inherit
Initial: 0
Applies to: all elements except table-row-group, table-header-group,
table-footer-group, table-row, table-column-group and table-column
Inherited: no
Percentages: refer to width of containing block
Media: visual
Computed value: the percentage as specified or the absolute length
These properties set the top, right, bottom, and left padding of a
box.
Example(s):
blockquote { padding-top: 0.3em }
'padding'
Value: {1,4} | inherit
Initial: see individual properties
Applies to: all elements except table-row-group, table-header-group,
table-footer-group, table-row, table-column-group and table-column
Inherited: no
Percentages: refer to width of containing block
Media: visual
Computed value: see individual properties
The 'padding' property is a shorthand property for setting
'padding-top', 'padding-right', 'padding-bottom', and 'padding-left'
at the same place in the style sheet.
If there is only one value, it applies to all sides. If there are two
values, the top and bottom paddings are set to the first value and the
right and left paddings are set to the second. If there are three
values, the top is set to the first value, the left and right are set
to the second, and the bottom is set to the third. If there are four
values, they apply to the top, right, bottom, and left, respectively.
The surface color or image of the padding area is specified via the
'background' property:
Example(s):
h1 {
background: white;
padding: 1em 2em;
}
The example above specifies a '1em' vertical padding ('padding-top'
and 'padding-bottom') and a '2em' horizontal padding ('padding-right'
and 'padding-left'). The 'em' unit is relative to the element's font
size: '1em' is equal to the size of the font in use.
8.5 Border properties
The border properties specify the width, color, and style of the
border area of a box. These properties apply to all elements.
Note. Notably for HTML, user agents may render borders for certain
user interface elements (e.g., buttons, menus, etc.) differently than
for "ordinary" elements.
8.5.1 Border width: 'border-top-width', 'border-right-width',
'border-bottom-width', 'border-left-width', and 'border-width'
The border width properties specify the width of the border area. The
properties defined in this section refer to the value
type, which may take one of the following values:
thin
A thin border.
medium
A medium border.
thick
A thick border.
The border's thickness has an explicit value. Explicit border
widths cannot be negative.
The interpretation of the first three values depends on the user
agent. The following relationships must hold, however:
'thin' <='medium' <= 'thick'.
Furthermore, these widths must be constant throughout a document.
'border-top-width', 'border-right-width', 'border-bottom-width',
'border-left-width'
Value: | inherit
Initial: medium
Applies to: all elements
Inherited: no
Percentages: N/A
Media: visual
Computed value: absolute length; '0' if the border style is 'none'
or 'hidden'
These properties set the width of the top, right, bottom, and left
border of a box.
'border-width'
Value: {1,4} | inherit
Initial: see individual properties
Applies to: all elements
Inherited: no
Percentages: N/A
Media: visual
Computed value: see individual properties
This property is a shorthand property for setting 'border-top-width',
'border-right-width', 'border-bottom-width', and 'border-left-width'
at the same place in the style sheet.
If there is only one value, it applies to all sides. If there are two
values, the top and bottom borders are set to the first value and the
right and left are set to the second. If there are three values, the
top is set to the first value, the left and right are set to the
second, and the bottom is set to the third. If there are four values,
they apply to the top, right, bottom, and left, respectively.
Example(s):
In the examples below, the comments indicate the resulting widths of
the top, right, bottom, and left borders:
h1 { border-width: thin } /* thin thin thin thin */
h1 { border-width: thin thick } /* thin thick thin thick */
h1 { border-width: thin thick medium } /* thin thick medium thick */
8.5.2 Border color: 'border-top-color', 'border-right-color',
'border-bottom-color', 'border-left-color', and 'border-color'
The border color properties specify the color of a box's border.
'border-top-color', 'border-right-color', 'border-bottom-color',
'border-left-color'
Value: | transparent | inherit
Initial: the value of the 'color' property
Applies to: all elements
Inherited: no
Percentages: N/A
Media: visual
Computed value: when taken from the 'color' property, the computed
value of 'color'; otherwise, as specified
'border-color'
Value: [ | transparent ]{1,4} | inherit
Initial: see individual properties
Applies to: all elements
Inherited: no
Percentages: N/A
Media: visual
Computed value: see individual properties
The 'border-color' property sets the color of the four borders. Values
have the following meanings:
Specifies a color value.
transparent
The border is transparent (though it may have width).
The 'border-color' property can have from one to four values, and the
values are set on the different sides as for 'border-width'.
If an element's border color is not specified with a border property,
user agents must use the value of the element's 'color' property as
the computed value for the border color.
Example(s):
In this example, the border will be a solid black line.
p {
color: black;
background: white;
border: solid;
}
8.5.3 Border style: 'border-top-style', 'border-right-style',
'border-bottom-style', 'border-left-style', and 'border-style'
The border style properties specify the line style of a box's border
(solid, double, dashed, etc.). The properties defined in this section
refer to the value type, which may take one of the
following values:
none
No border; the computed border width is zero.
hidden
Same as 'none', except in terms of border conflict resolution
for table elements.
dotted
The border is a series of dots.
dashed
The border is a series of short line segments.
solid
The border is a single line segment.
double
The border is two solid lines. The sum of the two lines and the
space between them equals the value of 'border-width'.
groove
The border looks as though it were carved into the canvas.
ridge
The opposite of 'groove': the border looks as though it were
coming out of the canvas.
inset
The border makes the box look as though it were embedded in the
canvas.
outset
The opposite of 'inset': the border makes the box look as
though it were coming out of the canvas.
All borders are drawn on top of the box's background. The color of
borders drawn for values of 'groove', 'ridge', 'inset', and 'outset'
depends on the element's border color properties, but UAs may choose
their own algorithm to calculate the actual colors used. For instance,
if the 'border-color' has the value 'silver', then a UA could use a
gradient of colors from white to dark gray to indicate a sloping
border.
'border-top-style', 'border-right-style', 'border-bottom-style',
'border-left-style'
Value: | inherit
Initial: none
Applies to: all elements
Inherited: no
Percentages: N/A
Media: visual
Computed value: as specified
'border-style'
Value: {1,4} | inherit
Initial: see individual properties
Applies to: all elements
Inherited: no
Percentages: N/A
Media: visual
Computed value: see individual properties
The 'border-style' property sets the style of the four borders. It can
have from one to four values, and the values are set on the different
sides as for 'border-width' above.
Example(s):
#xy34 { border-style: solid dotted }
In the above example, the horizontal borders will be 'solid' and the
vertical borders will be 'dotted'.
Since the initial value of the border styles is 'none', no borders
will be visible unless the border style is set.
8.5.4 Border shorthand properties: 'border-top', 'border-right',
'border-bottom', 'border-left', and 'border'
'border-top', 'border-right', 'border-bottom', 'border-left'
Value: [ || || ]
| inherit
Initial: see individual properties
Applies to: all elements
Inherited: no
Percentages: N/A
Media: visual
Computed value: see individual properties
This is a shorthand property for setting the width, style, and color
of the top, right, bottom, and left border of a box.
Example(s):
h1 { border-bottom: thick solid red }
The above rule will set the width, style, and color of the border
below the H1 element. Omitted values are set to their initial values.
Since the following rule does not specify a border color, the border
will have the color specified by the 'color' property:
H1 { border-bottom: thick solid }
'border'
Value: [ || || ]
| inherit
Initial: see individual properties
Applies to: all elements
Inherited: no
Percentages: N/A
Media: visual
Computed value: see individual properties
The 'border' property is a shorthand property for setting the same
width, color, and style for all four borders of a box. Unlike the
shorthand 'margin' and 'padding' properties, the 'border' property
cannot set different values on the four borders. To do so, one or more
of the other border properties must be used.
Example(s):
For example, the first rule below is equivalent to the set of four
rules shown after it:
p { border: solid red }
p {
border-top: solid red;
border-right: solid red;
border-bottom: solid red;
border-left: solid red
}
Since, to some extent, the properties have overlapping functionality,
the order in which the rules are specified is important.
Example(s):
Consider this example:
blockquote {
border: solid red;
border-left: double;
color: black;
}
In the above example, the color of the left border is black, while the
other borders are red. This is due to 'border-left' setting the width,
style, and color. Since the color value is not given by the
'border-left' property, it will be taken from the 'color' property.
The fact that the 'color' property is set after the 'border-left'
property is not relevant.
8.6 The box model for inline elements in bidirection context
For each line box, UAs must take the inline boxes generated for each
element and render the margins, borders and padding in visual order
(not logical order).
When the element's 'direction' property is 'ltr', the left-most
generated box of the first line box in which the element appears has
the left margin, left border and left padding, and the right-most
generated box of the last line box in which the element appears has
the right padding, right border and right margin.
When the element's 'direction' property is 'rtl', the right-most
generated box of the first line box in which the element appears has
the right padding, right border and right margin, and the left-most
generated box of the last line box in which the element appears has
the left margin, left border and left padding.
_________________________________________________________________
_________________________________________________________________
9 Visual formatting model
Contents
* 9.1 Introduction to the visual formatting model
+ 9.1.1 The viewport
+ 9.1.2 Containing blocks
* 9.2 Controlling box generation
+ 9.2.1 Block-level elements and block boxes
o 9.2.1.1 Anonymous block boxes
+ 9.2.2 Inline-level elements and inline boxes
o 9.2.2.1 Anonymous inline boxes
+ 9.2.3 Run-in boxes
+ 9.2.4 The 'display' property
* 9.3 Positioning schemes
+ 9.3.1 Choosing a positioning scheme: 'position' property
+ 9.3.2 Box offsets: 'top', 'right', 'bottom', 'left'
* 9.4 Normal flow
+ 9.4.1 Block formatting contexts
+ 9.4.2 Inline formatting context
+ 9.4.3 Relative positioning
* 9.5 Floats
+ 9.5.1 Positioning the float: the 'float' property
+ 9.5.2 Controlling flow next to floats: the 'clear' property
* 9.6 Absolute positioning
+ 9.6.1 Fixed positioning
* 9.7 Relationships between 'display', 'position', and 'float'
* 9.8 Comparison of normal flow, floats, and absolute positioning
+ 9.8.1 Normal flow
+ 9.8.2 Relative positioning
+ 9.8.3 Floating a box
+ 9.8.4 Absolute positioning
* 9.9 Layered presentation
+ 9.9.1 Specifying the stack level: the 'z-index' property
* 9.10 Text direction: the 'direction' and 'unicode-bidi' properties
9.1 Introduction to the visual formatting model
This chapter and the next describe the visual formatting model: how
user agents process the document tree for visual media.
In the visual formatting model, each element in the document tree
generates zero or more boxes according to the box model. The layout of
these boxes is governed by:
* box dimensions and type.
* positioning scheme (normal flow, float, and absolute positioning).
* relationships between elements in the document tree.
* external information (e.g., viewport size, intrinsic dimensions of
images, etc.).
The properties defined in this chapter and the next apply to both
continuous media and paged media. However, the meanings of the margin
properties vary when applied to paged media (see the page model for
details).
The visual formatting model does not specify all aspects of formatting
(e.g., it does not specify a letter-spacing algorithm). Conforming
user agents may behave differently for those formatting issues not
covered by this specification.
9.1.1 The viewport
User agents for continuous media generally offer users a viewport (a
window or other viewing area on the screen) through which users
consult a document. User agents may change the document's layout when
the viewport is resized (see the initial containing block).
When the viewport is smaller than the area of the canvas on which the
document is rendered, the user agent should offer a scrolling
mechanism. There is at most one viewport per canvas, but user agents
may render to more than one canvas (i.e., provide different views of
the same document).
9.1.2 Containing blocks
In CSS 2.1, many box positions and sizes are calculated with respect
to the edges of a rectangular box called a containing block. In
general, generated boxes act as containing blocks for descendant
boxes; we say that a box "establishes" the containing block for its
descendants. The phrase "a box's containing block" means "the
containing block in which the box lives," not the one it generates.
Each box is given a position with respect to its containing block, but
it is not confined by this containing block; it may overflow.
The details of how a containing block's dimensions are calculated are
described in the next chapter.
9.2 Controlling box generation
The following sections describe the types of boxes that may be
generated in CSS 2.1. A box's type affects, in part, its behavior in
the visual formatting model. The 'display' property, described below,
specifies a box's type.
9.2.1 Block-level elements and block boxes
Block-level elements are those elements of the source document that
are formatted visually as blocks (e.g., paragraphs). Several values of
the 'display' property make an element block-level: 'block',
'list-item', and 'run-in' (part of the time; see run-in boxes), and
'table'.
Block-level elements (except for display 'table' elements, which are
described in a later chapter) generate a principal block box that
contains either only block boxes or only inline boxes. The principal
block box establishes the containing block for descendant boxes and
generated content and is also the box involved in any positioning
scheme. Principal block boxes participate in a block formatting
context.
Some block-level elements generate additional boxes outside of the
principal box: 'list-item' elements. These additional boxes are placed
with respect to the principal box.
9.2.1.1 Anonymous block boxes
In a document like this:

Some text

More text

(and assuming the DIV and the P both have 'display: block'), the DIV
appears to have both inline content and block content. To make it
easier to define the formatting, we assume that there is an anonymous
block box around "Some text".
diagram showing the three boxes for the example above [D]
Diagram showing the three boxes, of which one is anonymous, for the
example above.
In other words: if a block box (such as that generated for the DIV
above) has another block box or run-in box inside it (such as the P
above), then we force it to have only block boxes and run-in boxes
inside it.
When an inline box contains a block box, the inline box (and its
inline ancestors within the same line box) are broken around the
block. The line boxes before the break and after the break are
enclosed in anonymous boxes, and the block box becomes a sibling of
those anonymous boxes. When such an inline box is affected by relative
positioning, the relative positioning also affects the block box.
Example(s):
This model would apply in the following example if the following
rules:
body { display: inline }
p { display: block }
were used with this HTML document:
Anonymous text interrupted by a block
This is anonymous text before the P.

This is the content of P.

This is anonymous text after the P.
The BODY element contains a chunk (C1) of anonymous text followed by a
block-level element followed by another chunk (C2) of anonymous text.
The resulting boxes would be an anonymous block box around the BODY,
containing an anonymous block box around C1, the P block box, and
another anonymous block box around C2.
The properties of anonymous boxes are inherited from the enclosing
non-anonymous box (e.g. in the example just below the subsection
heading "Anonymous block boxes", the one for DIV). Non-inherited
properties have their initial value. For example, the font of the
anonymous box is inherited from the DIV, but the margins will be 0.
Properties set on elements that cause anonymous block boxes to be
generated still apply to the boxes and content of that element. For
example, if a border had been set on the BODY element in the above
example, the border would be drawn around C1 (open at the end of the
line) and C2 (open at the start of the line).
Some user agents have implemented borders on inlines containing blocks
in other ways, e.g. by wrapping such nested blocks inside "anonymous
line boxes" and thus drawing inline borders around such boxes. As CSS1
and CSS2 did not define this behavior, CSS1-only and CSS2-only user
agents may implement this alternative model and still claim
conformance to this part of CSS 2.1. This does not apply to UAs
developed after this specification was released.
9.2.2 Inline-level elements and inline boxes
Inline-level elements are those elements of the source document that
do not form new blocks of content; the content is distributed in lines
(e.g., emphasized pieces of text within a paragraph, inline images,
etc.). Several values of the 'display' property make an element
inline: 'inline', 'inline-table', and 'run-in' (part of the time; see
run-in boxes). Inline-level elements generate inline boxes.
9.2.2.1 Anonymous inline boxes
In a document with HTML markup like this:

Some emphasized text

The

generates a block box, with several inline boxes inside it.
The box for "emphasized" is an inline box generated by an inline
element (), but the other boxes ("Some" and "text") are inline
boxes generated by a block-level element (

). The latter are called
anonymous inline boxes, because they don't have an associated
inline-level element.
Such anonymous inline boxes inherit inheritable properties from their
block parent box. Non-inherited properties have their initial value.
In the example, the color of the anonymous inline boxes is inherited
from the P, but the background is transparent.
Whitespace content that would subsequently be collapsed away according
to the 'white-space' property does not generate any anonymous inline
boxes.
If it is clear from the context which type of anonymous box is meant,
both anonymous inline boxes and anonymous block boxes are simply
called anonymous boxes in this specification.
There are more types of anonymous boxes that arise when formatting
tables.
9.2.3 Run-in boxes
A run-in box behaves as follows:
1. If the run-in box contains a block box, the run-in box becomes a
block box.
2. If a sibling block box (that does not float and is not absolutely
positioned) follows the run-in box, the run-in box becomes the
first inline box of the block box. A run-in cannot run in to a
block that already starts with a run-in or that itself is a
run-in.
3. Otherwise, the run-in box becomes a block box.
A 'run-in' box is useful for run-in headers, as in this example:

A run-in box example

A run-in heading.

And a paragraph of text that
follows it.
This example might be formatted as:
A run-in heading. And a
paragraph of text that
follows it.
Despite appearing visually part of the following block box, a run-in
element still inherits properties from its parent in the source tree.
Please consult the section on generated content for information about
how run-in boxes interact with generated content.
9.2.4 The 'display' property
'display'
Value: inline | block | list-item | run-in | inline-block | table |
inline-table | table-row-group | table-header-group |
table-footer-group | table-row | table-column-group | table-column |
table-cell | table-caption | none | inherit
Initial: inline
Applies to: all elements
Inherited: no
Percentages: N/A
Media: all
Computed value: see text
The values of this property have the following meanings:
block
This value causes an element to generate a block box.
inline-block
This value causes an element to generate a block box, which
itself is flowed as a single inline box, similar to a replaced
element. The inside of an inline-block is formatted as a block
box, and the element itself is formatted as an inline replaced
element.
inline
This value causes an element to generate one or more inline
boxes.
list-item
This value causes an element (e.g., LI in HTML) to generate a
principal block box and a list-item inline box. For information
about lists and examples of list formatting, please consult the
section on lists.
none
This value causes an element to generate no boxes in the
formatting structure (i.e., the element has no effect on
layout). Descendant elements do not generate any boxes either;
this behavior cannot be overridden by setting the 'display'
property on the descendants.
Please note that a display of 'none' does not create an
invisible box; it creates no box at all. CSS includes
mechanisms that enable an element to generate boxes in the
formatting structure that affect formatting but are not visible
themselves. Please consult the section on visibility for
details.
run-in
This value creates either block or inline boxes, depending on
context. Properties apply to run-in boxes based on their final
status (inline-level or block-level).
table, inline-table, table-row-group, table-column,
table-column-group, table-header-group, table-footer-group,
table-row, table-cell, and table-caption
These values cause an element to behave like a table element
(subject to restrictions described in the chapter on tables).
The computed value is the same as the specified value, except for
positioned and floating elements (see Relationships between 'display',
'position', and 'float') and for the root element. For the root
element, the computed value is changed as described in the section on
the relationships between 'display', 'position', and 'float'.
Note that although the initial value of 'display' is 'inline', rules
in the user agent's default style sheet may override this value. See
the sample style sheet for HTML 4 in the appendix.
Example(s):
Here are some examples of the 'display' property:
p { display: block }
em { display: inline }
li { display: list-item }
img { display: none } /* Don't display images */
9.3 Positioning schemes
In CSS 2.1, a box may be laid out according to three positioning
schemes:
1. Normal flow. In CSS 2.1, normal flow includes block formatting of
block boxes, inline formatting of inline boxes, relative
positioning of block or inline boxes, and positioning of run-in
boxes.
2. Floats. In the float model, a box is first laid out according to
the normal flow, then taken out of the flow and shifted to the
left or right as far as possible. Content may flow along the side
of a float.
3. Absolute positioning. In the absolute positioning model, a box is
removed from the normal flow entirely (it has no impact on later
siblings) and assigned a position with respect to a containing
block.
Note. CSS 2.1's positioning schemes help authors make their documents
more accessible by allowing them to avoid mark-up tricks (e.g.,
invisible images) used for layout effects.
9.3.1 Choosing a positioning scheme: 'position' property
The 'position' and 'float' properties determine which of the CSS 2.1
positioning algorithms is used to calculate the position of a box.
'position'
Value: static | relative | absolute | fixed | inherit
Initial: static
Applies to: all elements
Inherited: no
Percentages: N/A
Media: visual
Computed value: as specified
The values of this property have the following meanings:
static
The box is a normal box, laid out according to the normal flow.
The 'top', 'right', 'bottom', and 'left' properties do not
apply.
relative
The box's position is calculated according to the normal flow
(this is called the position in normal flow). Then the box is
offset relative to its normal position. When a box B is
relatively positioned, the position of the following box is
calculated as though B were not offset. The effect of
'position:relative' on table-row-group, table-header-group,
table-footer-group, table-row, table-column-group,
table-column, table-cell, and table-caption elements is
undefined.
absolute
The box's position (and possibly size) is specified with the
'top', 'right', 'bottom', and 'left' properties. These
properties specify offsets with respect to the box's containing
block. Absolutely positioned boxes are taken out of the normal
flow. This means they have no impact on the layout of later
siblings. Also, though absolutely positioned boxes have
margins, they do not collapse with any other margins.
fixed
The box's position is calculated according to the 'absolute'
model, but in addition, the box is fixed with respect to some
reference. As with the 'absolute' model, the box's margins do
not collapse with any other margins. In the case of handheld,
projection, screen, tty, and tv media types, the box is fixed
with respect to the viewport and doesn't move when scrolled. In
the case of the print media type, the box is rendered on every
page, and is fixed with respect to the page box, even if the
page is seen through a viewport (in the case of a
print-preview, for example). For other media types, the
presentation is undefined. Authors may wish to specify 'fixed'
in a media-dependent way. For instance, an author may want a
box to remain at the top of the viewport on the screen, but not
at the top of each printed page. The two specifications may be
separated by using an @media rule, as in:
Example(s):
@media screen {
h1#first { position: fixed }
}
@media print {
h1#first { position: static }
}
UAs must not paginate the content of fixed boxes. Note that UAs
may print invisible content in other ways. See "Content outside
the page box" in chapter 13.
User agents may treat position as 'static' on the root element.
9.3.2 Box offsets: 'top', 'right', 'bottom', 'left'
An element is said to be positioned if its 'position' property has a
value other than 'static'. Positioned elements generate positioned
boxes, laid out according to four properties:
'top'
Value: | | auto | inherit
Initial: auto
Applies to: positioned elements
Inherited: no
Percentages: refer to height of containing block
Media: visual
Computed value: for 'position:relative', see section Relative
Positioning. For 'position:static', 'auto'. Otherwise: if specified as
a length, the corresponding absolute length; if specified as a
percentage, the specified value; otherwise, 'auto'.
This property specifies how far an absolutely positioned box's top
margin edge is offset below the top edge of the box's containing
block. For relatively positioned boxes, the offset is with respect to
the top edges of the box itself (i.e., the box is given a position in
the normal flow, then offset from that position according to these
properties). Note: For absolutely positioned elements whose containing
block is based on a block-level element, this property is an offset
from the padding edge of that element.
'right'
Value: | | auto | inherit
Initial: auto
Applies to: positioned elements
Inherited: no
Percentages: refer to width of containing block
Media: visual
Computed value: for 'position:relative', see section Relative
Positioning. For 'position:static', 'auto'. Otherwise: if specified as
a length, the corresponding absolute length; if specified as a
percentage, the specified value; otherwise, 'auto'.
Like 'top', but specifies how far a box's right margin edge is offset
to the left of the right edge of the box's containing block. For
relatively positioned boxes, the offset is with respect to the right
edge of the box itself. Note: For absolutely positioned elements whose
containing block is based on a block-level element, this property is
an offset from the padding edge of that element.
'bottom'
Value: | | auto | inherit
Initial: auto
Applies to: positioned elements
Inherited: no
Percentages: refer to height of containing block
Media: visual
Computed value: for 'position:relative', see section Relative
Positioning. For 'position:static', 'auto'. Otherwise: if specified as
a length, the corresponding absolute length; if specified as a
percentage, the specified value; otherwise, 'auto'.
Like 'top', but specifies how far a box's bottom margin edge is offset
above the bottom of the box's containing block. For relatively
positioned boxes, the offset is with respect to the bottom edge of the
box itself. Note: For absolutely positioned elements whose containing
block is based on a block-level element, this property is an offset
from the padding edge of that element.
'left'
Value: | | auto | inherit
Initial: auto
Applies to: positioned elements
Inherited: no
Percentages: refer to width of containing block
Media: visual
Computed value: for 'position:relative', see section Relative
Positioning. For 'position:static', 'auto'. Otherwise: if specified as
a length, the corresponding absolute length; if specified as a
percentage, the specified value; otherwise, 'auto'.
Like 'top', but specifies how far a box's left margin edge is offset
to the right of the left edge of the box's containing block. For
relatively positioned boxes, the offset is with respect to the left
edge of the box itself. Note: For absolutely positioned elements whose
containing block is based on a block-level element, this property is
an offset from the padding edge of that element.
The values for the four properties have the following meanings:
The offset is a fixed distance from the reference edge.
Negative values are allowed.
The offset is a percentage of the containing block's width (for
'left' or 'right') or height (for 'top' and 'bottom'). Negative
values are allowed.
auto
For non-replaced elements, the effect of this value depends on
which of related properties have the value 'auto' as well. See
the sections on the width and height of absolutely positioned,
non-replaced elements for details. For replaced elements, the
effect of this value depends only on the intrinsic dimensions
of the replaced content. See the sections on the width and
height of absolutely positioned, replaced elements for details.
9.4 Normal flow
Boxes in the normal flow belong to a formatting context, which may be
block or inline, but not both simultaneously. Block boxes participate
in a block formatting context. Inline boxes participate in an inline
formatting context.
9.4.1 Block formatting contexts
Floats, absolutely positioned elements, inline-blocks, table-cells,
table-captions, and elements with 'overflow' other than 'visible'
(except when that value has been propagated to the viewport) establish
new block formatting contexts.
In a block formatting context, boxes are laid out one after the other,
vertically, beginning at the top of a containing block. The vertical
distance between two sibling boxes is determined by the 'margin'
properties. Vertical margins between adjacent block boxes in a block
formatting context collapse.
In a block formatting context, each box's left outer edge touches the
left edge of the containing block (for right-to-left formatting, right
edges touch). This is true even in the presence of floats (although a
box's line boxes may shrink due to the floats), unless the box
establishes a new block formatting context (in which case the box
itself may become narrower due to the floats).
For information about page breaks in paged media, please consult the
section on allowed page breaks.
9.4.2 Inline formatting context
In an inline formatting context, boxes are laid out horizontally, one
after the other, beginning at the top of a containing block.
Horizontal margins, borders, and padding are respected between these
boxes. The boxes may be aligned vertically in different ways: their
bottoms or tops may be aligned, or the baselines of text within them
may be aligned. The rectangular area that contains the boxes that form
a line is called a line box.
The width of a line box is determined by a containing block and the
presence of floats. The height of a line box is determined by the
rules given in the section on line height calculations.
A line box is always tall enough for all of the boxes it contains.
However, it may be taller than the tallest box it contains (if, for
example, boxes are aligned so that baselines line up). When the height
of a box B is less than the height of the line box containing it, the
vertical alignment of B within the line box is determined by the
'vertical-align' property. When several inline boxes cannot fit
horizontally within a single line box, they are distributed among two
or more vertically-stacked line boxes. Thus, a paragraph is a vertical
stack of line boxes. Line boxes are stacked with no vertical
separation and they never overlap.
In general, the left edge of a line box touches the left edge of its
containing block and the right edge touches the right edge of its
containing block. However, floating boxes may come between the
containing block edge and the line box edge. Thus, although line boxes
in the same inline formatting context generally have the same width
(that of the containing block), they may vary in width if available
horizontal space is reduced due to floats. Line boxes in the same
inline formatting context generally vary in height (e.g., one line
might contain a tall image while the others contain only text).
When the total width of the inline boxes on a line is less than the
width of the line box containing them, their horizontal distribution
within the line box is determined by the 'text-align' property. If
that property has the value 'justify', the user agent may stretch
spaces and words in inline boxes (except for inline-table and
inline-block boxes) as well.
When an inline box exceeds the width of a line box, it is split into
several boxes and these boxes are distributed across several line
boxes. If an inline box cannot be split (e.g. if the inline box
contains a single character, or language specific word breaking rules
disallow a break within the inline box, or if the inline box is
affected by a white-space value of nowrap or pre), then the inline box
overflows the line box.
When an inline box is split, margins, borders, and padding have no
visual effect where the split occurs (or at any split, when there are
several).
Inline boxes may also be split into several boxes within the same line
box due to bidirectional text processing.
Line boxes that contain no text, no preserved white space, no inline
elements with non-zero margins, padding, or borders, and no other
in-flow content (such as images, inline blocks or inline tables), and
don't end with a line feed must be treated as zero-height line boxes.
For the purposes of margin collapsing, this line box must be ignored.
Here is an example of inline box construction. The following paragraph
(created by the HTML block-level element P) contains anonymous text
interspersed with the elements EM and STRONG:

Several emphasized words appear
in this sentence, dear.

The P element generates a block box that contains five inline boxes,
three of which are anonymous:
* Anonymous: "Several"
* EM: "emphasized words"
* Anonymous: "appear"
* STRONG: "in this"
* Anonymous: "sentence, dear."
To format the paragraph, the user agent flows the five boxes into line
boxes. In this example, the box generated for the P element
establishes the containing block for the line boxes. If the containing
block is sufficiently wide, all the inline boxes will fit into a
single line box:
Several emphasized words appear in this sentence, dear.
If not, the inline boxes will be split up and distributed across
several line boxes. The previous paragraph might be split as follows:
Several emphasized words appear
in this sentence, dear.
or like this:
Several emphasized
words appear in this
sentence, dear.
In the previous example, the EM box was split into two EM boxes (call
them "split1" and "split2"). Margins, borders, padding, or text
decorations have no visible effect after split1 or before split2.
Consider the following example:
Example of inline flow on several lines

Several emphasized words appear here.

Depending on the width of the P, the boxes may be distributed as
follows:
Image illustrating the effect of line breaking on the display of
margins, borders, and padding. [D]
* The margin is inserted before "emphasized" and after "words".
* The padding is inserted before, above, and below "emphasized" and
after, above, and below "words". A dashed border is rendered on
three sides in each case.
9.4.3 Relative positioning
Once a box has been laid out according to the normal flow or floated,
it may be shifted relative to this position. This is called relative
positioning. Offsetting a box (B1) in this way has no effect on the
box (B2) that follows: B2 is given a position as if B1 were not offset
and B2 is not re-positioned after B1's offset is applied. This implies
that relative positioning may cause boxes to overlap. However, if
relative positioning causes an 'overflow:auto' or 'overflow:scroll'
box to have overflow, the UA must allow the user to access this
content (at its offset position), which, through the creation of
scrollbars, may affect layout.
A relatively positioned box keeps its normal flow size, including line
breaks and the space originally reserved for it. The section on
containing blocks explains when a relatively positioned box
establishes a new containing block.
For relatively positioned elements, 'left' and 'right' move the
box(es) horizontally, without changing their size. 'left' moves the
boxes to the right, and 'right' moves them to the left. Since boxes
are not split or stretched as a result of 'left' or 'right', the
computed values are always: left = -right.
If both 'left' and 'right' are 'auto' (their initial values), the
computed values are '0' (i.e., the boxes stay in their original
position).
If 'left' is 'auto', its computed value is minus the value of 'right'
(i.e., the boxes move to the left by the value of 'right').
If 'right' is specified as 'auto', its computed value is minus the
value of 'left'.
If neither 'left' nor 'right' is 'auto', the position is
over-constrained, and one of them has to be ignored. If the
'direction' property of the containing block is 'ltr, the value of
'left' wins and 'right' becomes -'left'. If 'direction' of the
containing block is 'rtl', 'right' wins and 'left' is ignored.
Example(s):
Example. The following three rules are equivalent:
div.a8 { position: relative; direction: ltr; left: -1em; right: auto }
div.a8 { position: relative; direction: ltr; left: auto; right: 1em }
div.a8 { position: relative; direction: ltr; left: -1em; right: 5em }
The 'top' and 'bottom' properties move relatively positioned
element(s) up or down without changing their size. 'top' moves the
boxes down, and 'bottom' moves them up. Since boxes are not split or
stretched as a result of 'top' or 'bottom', the computed values are
always: top = -bottom. If both are 'auto', their computed values are
both '0'. If one of them is 'auto', it becomes the negative of the
other. If neither is 'auto', 'bottom' is ignored (i.e., the computed
value of 'bottom' will be minus the value of 'top').
Note. Dynamic movement of relatively positioned boxes can produce
animation effects in scripting environments (see also the 'visibility'
property). Although relative positioning may be used as a form of
superscripting and subscripting, the line height is not automatically
adjusted to take the positioning into consideration. See the
description of line height calculations for more information.
Examples of relative positioning are provided in the section comparing
normal flow, floats, and absolute positioning.
9.5 Floats
A float is a box that is shifted to the left or right on the current
line. The most interesting characteristic of a float (or "floated" or
"floating" box) is that content may flow along its side (or be
prohibited from doing so by the 'clear' property). Content flows down
the right side of a left-floated box and down the left side of a
right-floated box. The following is an introduction to float
positioning and content flow; the exact rules governing float behavior
are given in the description of the 'float' property.
A floated box is shifted to the left or right until its outer edge
touches the containing block edge or the outer edge of another float.
If there's a line box, the top of the floated box is aligned with the
top of the current line box.
If there isn't enough horizontal room for the float, it is shifted
downward until either it fits or there are no more floats present.
Since a float is not in the flow, non-positioned block boxes created
before and after the float box flow vertically as if the float didn't
exist. However, line boxes created next to the float are shortened to
make room for margin box of the float. If a shortened line box is too
small to contain any further content, then it is shifted downward
until either it fits or there are no more floats present. Any content
in the current line before a floated box is reflowed in the first
available line on the other side of the float. In other words, if
inline boxes are placed on the line before a left float is encountered
that fits in the remaining line box space, the left float is placed on
that line, aligned with the top of the line box, and then the inline
boxes already on the line are moved accordingly to the right of the
float (the right being the other side of the left float) and vice
versa for rtl and right floats.
The border box of a table, a block-level replaced element, or an
element in the normal flow that establishes a new block formatting
context (such as an element with 'overflow' other than 'visible') must
not overlap any floats in the same block formatting context as the
element itself. If necessary, implementations should clear the said
element by placing it below any preceding floats, but may place it
adjacent to such floats if there is sufficient space. They may even
make the border box of said element narrower than defined by
section 10.3.3. CSS2 does not define when a UA may put said element
next to the float or by how much said element may become narrower.
Example(s):
Example. In the following document fragment, the containing block is
too narrow to contain the content next to the float, so the content
gets moved to below the floats where it is aligned in the line box
according to the text-align property.
p { width: 10em; border: solid aqua; }
span { float: left; width: 5em; height: 5em; border: solid blue; }
...

Supercalifragilisticexpialidocious

This fragment might look like this:
Image illustrating the effect of an unbreakable piece of content being
reflowed to just after a float which left insufficient room next to it
for the content to fit.
Several floats may be adjacent, and this model also applies to
adjacent floats in the same line.
Example(s):
The following rule floats all IMG boxes with class="icon" to the left
(and sets the left margin to '0'):
img.icon {
float: left;
margin-left: 0;
}
Consider the following HTML source and style sheet:
Float example

Some sample text that has no other...
The IMG box is floated to the left. The content that follows is
formatted to the right of the float, starting on the same line as the
float. The line boxes to the right of the float are shortened due to
the float's presence, but resume their "normal" width (that of the
containing block established by the P element) after the float. This
document might be formatted as:
Image illustrating how floating boxes interact with margins. [D]
Formatting would have been exactly the same if the document had been:

Some sample text
that has no other...
because the content to the left of the float is displaced by the float
and reflowed down its right side.
As stated in section 8.3.1, the margins of floating boxes never
collapse with margins of adjacent boxes. Thus, in the previous
example, vertical margins do not collapse between the P box and the
floated IMG box.
The contents of floats are stacked as if floats generated new stacking
contexts, except that any elements that actually create new stacking
contexts take part in the float's parent's stacking context. A float
can overlap other boxes in the normal flow (e.g., when a normal flow
box next to a float has negative margins). When this happens, floats
are rendered in front of non-positioned in-flow blocks, but behind
in-flow inlines.
Example(s):
Here is another illustration, showing what happens when a float
overlaps borders of elements in the normal flow.
Image showing a floating image that overlaps the borders of two
paragraphs: the borders are interrupted by the image. [D]
A floating image obscures borders of block boxes it overlaps.
The following example illustrates the use of the 'clear' property to
prevent content from flowing next to a float.
Example(s):
Assuming a rule such as this:
p { clear: left }
formatting might look like this:
Image showing a floating image and the effect of 'clear: left' on the
two paragraphs. [D]
Both paragraphs have set 'clear: left', which causes the second
paragraph to be "pushed down" to a position below the float --
"clearance" is added above its top margin to accomplish this (see the
'clear' property).
9.5.1 Positioning the float: the 'float' property
'float'
Value: left | right | none | inherit
Initial: none
Applies to: all, but see 9.7
Inherited: no
Percentages: N/A
Media: visual
Computed value: as specified
This property specifies whether a box should float to the left, right,
or not at all. It may be set for any element, but only applies to
elements that generate boxes that are not absolutely positioned. The
values of this property have the following meanings:
left
The element generates a block box that is floated to the left.
Content flows on the right side of the box, starting at the top
(subject to the 'clear' property).
right
Similar to 'left', except the box is floated to the right, and
content flows on the left side of the box, starting at the top.
none
The box is not floated.
User agents may treat float as 'none' on the root element.
Here are the precise rules that govern the behavior of floats:
1. The left outer edge of a left-floating box may not be to the left
of the left edge of its containing block. An analogous rule holds
for right-floating elements.
2. If the current box is left-floating, and there are any
left-floating boxes generated by elements earlier in the source
document, then for each such earlier box, either the left outer
edge of the current box must be to the right of the right outer
edge of the earlier box, or its top must be lower than the bottom
of the earlier box. Analogous rules hold for right-floating boxes.
3. The right outer edge of a left-floating box may not be to the
right of the left outer edge of any right-floating box that is to
the right of it. Analogous rules hold for right-floating elements.
4. A floating box's outer top may not be higher than the top of its
containing block. When the float occurs between two collapsing
margins, the float is positioned as if it had an otherwise empty
anonymous block parent taking part in the flow. The position of
such a parent is defined by the rules in the section on margin
collapsing.
5. The outer top of a floating box may not be higher than the outer
top of any block or floated box generated by an element earlier in
the source document.
6. The outer top of an element's floating box may not be higher than
the top of any line-box containing a box generated by an element
earlier in the source document.
7. A left-floating box that has another left-floating box to its left
may not have its right outer edge to the right of its containing
block's right edge. (Loosely: a left float may not stick out at
the right edge, unless it is already as far to the left as
possible.) An analogous rule holds for right-floating elements.
8. A floating box must be placed as high as possible.
9. A left-floating box must be put as far to the left as possible, a
right-floating box as far to the right as possible. A higher
position is preferred over one that is further to the left/right.
References to other elements in these rules refer only to other
elements in the same block formatting context as the float.
Example(s):
This HTML fragment results in the b floating to the right.

ab

If the P element's width is enough, the a and the b will be side by
side. It might look like this:
An a at the left side of a box and a b at the right side
9.5.2 Controlling flow next to floats: the 'clear' property
'clear'
Value: none | left | right | both | inherit
Initial: none
Applies to: block-level elements
Inherited: no
Percentages: N/A
Media: visual
Computed value: as specified
This property indicates which sides of an element's box(es) may not be
adjacent to an earlier floating box. The 'clear' property does not
consider floats inside the element itself or in other block formatting
contexts.
For run-in boxes, this property applies to the final block box to
which the run-in box belongs.
Clearance is introduced as spacing above the margin-top of an element.
It is used to push the element vertically (typically downward), past
the float.
Values have the following meanings when applied to non-floating block
boxes:
left
The clearance of the generated box is set to the amount
necessary to place the top border edge below the bottom outer
edge of any left-floating boxes that resulted from elements
earlier in the source document.
right
The clearance of the generated box is set to the amount
necessary to place the top border edge below the bottom outer
edge of any right-floating boxes that resulted from elements
earlier in the source document.
both
The clearance of the generated box is set to the amount
necessary to place the top border edge below the bottom outer
edge of any right-floating and left-floating boxes that
resulted from elements earlier in the source document.
none
No constraint on the box's position with respect to floats.
Computing the clearance of an element on which 'clear' is set is done
by first determining the hypothetical position of the element's top
border edge within its parent block. This position is determined after
the top margin of the element has been collapsed with previous
adjacent margins (including the top margin of the parent block).
If this hypothetical position of the element's top border edge is not
past the relevant floats, then its clearance must be set to the
greater of:
1. The amount necessary to place the border edge of the block even
with the bottom outer edge of the lowest float that is to be
cleared.
2. The amount necessary to make the sum of the following equal to the
distance to which these margins collapsed when the hypothetical
position was calculated:
+ the margins collapsing above the clearance
+ the clearance itself
+ if the block's own margins collapse together: the block's top
margin
+ if the block's own margins do not collapse together: the
margins collapsing below the clearance
Note: The clearance can be negative.
When the property is set on floating elements, it results in a
modification of the rules for positioning the float. An extra
constraint (#10) is added:
* The top outer edge of the float must be below the bottom outer
edge of all earlier left-floating boxes (in the case of 'clear:
left'), or all earlier right-floating boxes (in the case of
'clear: right'), or both ('clear: both').
Note. This property applied to all elements in CSS1. Implementations
may therefore have supported this property on all elements. In CSS2
and CSS 2.1 the 'clear' property only applies to block-level elements.
Therefore authors should only use this property on block-level
elements. If an implementation does support clear on inline elements,
rather than setting a clearance as explained above, the implementation
should force a break and effectively insert one or more empty line
boxes (or shifting the new line box downward as described in section
9.5) to move the top of the cleared inline's line box to below the
respective floating box(es).
9.6 Absolute positioning
In the absolute positioning model, a box is explicitly offset with
respect to its containing block. It is removed from the normal flow
entirely (it has no impact on later siblings). An absolutely
positioned box establishes a new containing block for normal flow
children and absolutely (but not fixed) positioned descendants.
However, the contents of an absolutely positioned element do not flow
around any other boxes. They may obscure the contents of another box
(or be obscured themselves), depending on the stack levels of the
overlapping boxes.
References in this specification to an absolutely positioned element
(or its box) imply that the element's 'position' property has the
value 'absolute' or 'fixed'.
9.6.1 Fixed positioning
Fixed positioning is a subcategory of absolute positioning. The only
difference is that for a fixed positioned box, the containing block is
established by the viewport. For continuous media, fixed boxes do not
move when the document is scrolled. In this respect, they are similar
to fixed background images. For paged media, boxes with fixed
positions are repeated on every page. This is useful for placing, for
instance, a signature at the bottom of each page.
Authors may use fixed positioning to create frame-like presentations.
Consider the following frame layout:
Image illustrating a frame-like layout with position='fixed'. [D]
This might be achieved with the following HTML document and style
rules:
A frame document with CSS 2.1

...

...

...

...

9.7 Relationships between 'display', 'position', and 'float'
The three properties that affect box generation and layout --
'display', 'position', and 'float' -- interact as follows:
1. If 'display' has the value 'none', then 'position' and 'float' do
not apply. In this case, the element generates no box.
2. Otherwise, if 'position' has the value 'absolute' or 'fixed', the
box is absolutely positioned, the computed value of 'float' is
'none', and display is set according to the table below. The
position of the box will be determined by the 'top', 'right',
'bottom' and 'left' properties and the box's containing block.
3. Otherwise, if 'float' has a value other than 'none', the box is
floated and 'display' is set according to the table below.
4. Otherwise, if the element is the root element, 'display' is set
according to the table below.
5. Otherwise, the remaining 'display' property values apply as
specified.
Specified value Computed value
inline-table table
inline, run-in, table-row-group, table-column, table-column-group,
table-header-group, table-footer-group, table-row, table-cell,
table-caption, inline-block block
others same as specified
9.8 Comparison of normal flow, floats, and absolute positioning
To illustrate the differences between normal flow, relative
positioning, floats, and absolute positioning, we provide a series of
examples based on the following HTML:
Comparison of positioning schemes

Beginning of body contents.
Start of outer contents.
Inner contents.
End of outer contents.
End of body contents.

In this document, we assume the following rules:
body { display: block; font-size:12px; line-height: 200%;
width: 400px; height: 400px }
p { display: block }
span { display: inline }
The final positions of boxes generated by the outer and inner elements
vary in each example. In each illustration, the numbers to the left of
the illustration indicate the normal flow position of the
double-spaced (for clarity) lines.
Note. The diagrams in this section are illustrative and not to scale.
They are meant to highlight the differences between the various
positioning schemes in CSS 2.1, and are not intended to be reference
renderings of the examples given.
9.8.1 Normal flow
Consider the following CSS declarations for outer and inner that don't
alter the normal flow of boxes:
#outer { color: red }
#inner { color: blue }
The P element contains all inline content: anonymous inline text and
two SPAN elements. Therefore, all of the content will be laid out in
an inline formatting context, within a containing block established by
the P element, producing something like:
Image illustrating the normal flow of text between parent and sibling
boxes. [D]
9.8.2 Relative positioning
To see the effect of relative positioning, we specify:
#outer { position: relative; top: -12px; color: red }
#inner { position: relative; top: 12px; color: blue }
Text flows normally up to the outer element. The outer text is then
flowed into its normal flow position and dimensions at the end of line
1. Then, the inline boxes containing the text (distributed over three
lines) are shifted as a unit by '-12px' (upwards).
The contents of inner, as a child of outer, would normally flow
immediately after the words "of outer contents" (on line 1.5).
However, the inner contents are themselves offset relative to the
outer contents by '12px' (downwards), back to their original position
on line 2.
Note that the content following outer is not affected by the relative
positioning of outer.
Image illustrating the effects of relative positioning on a box's
content. [D]
Note also that had the offset of outer been '-24px', the text of outer
and the body text would have overlapped.
9.8.3 Floating a box
Now consider the effect of floating the inner element's text to the
right by means of the following rules:
#outer { color: red }
#inner { float: right; width: 130px; color: blue }
Text flows normally up to the inner box, which is pulled out of the
flow and floated to the right margin (its 'width' has been assigned
explicitly). Line boxes to the left of the float are shortened, and
the document's remaining text flows into them.
Image illustrating the effects of floating a box. [D]
To show the effect of the 'clear' property, we add a sibling element
to the example:
Comparison of positioning schemes II

Beginning of body contents.
Start of outer contents.
Inner contents. Sibling contents.
End of outer contents.
End of body contents.

The following rules:
#inner { float: right; width: 130px; color: blue }
#sibling { color: red }
cause the inner box to float to the right as before and the document's
remaining text to flow into the vacated space:
Image illustrating the effects of floating a box without setting the
clear property to control the flow of text around the box. [D]
However, if the 'clear' property on the sibling element is set to
'right' (i.e., the generated sibling box will not accept a position
next to floating boxes to its right), the sibling content begins to
flow below the float:
#inner { float: right; width: 130px; color: blue }
#sibling { clear: right; color: red }
Image illustrating the effects of floating an element with setting the
clear property to control the flow of text around the element. [D]
9.8.4 Absolute positioning
Finally, we consider the effect of absolute positioning. Consider the
following CSS declarations for outer and inner:
#outer {
position: absolute;
top: 200px; left: 200px;
width: 200px;
color: red;
}
#inner { color: blue }
which cause the top of the outer box to be positioned with respect to
its containing block. The containing block for a positioned box is
established by the nearest positioned ancestor (or, if none exists,
the initial containing block, as in our example). The top side of the
outer box is '200px' below the top of the containing block and the
left side is '200px' from the left side. The child box of outer is
flowed normally with respect to its parent.
Image illustrating the effects of absolutely positioning a box. [D]
The following example shows an absolutely positioned box that is a
child of a relatively positioned box. Although the parent outer box is
not actually offset, setting its 'position' property to 'relative'
means that its box may serve as the containing block for positioned
descendants. Since the outer box is an inline box that is split across
several lines, the first inline box's top and left edges (depicted by
thick dashed lines in the illustration below) serve as references for
'top' and 'left' offsets.
#outer {
position: relative;
color: red
}
#inner {
position: absolute;
top: 200px; left: -100px;
height: 130px; width: 130px;
color: blue;
}
This results in something like the following:
Image illustrating the effects of absolutely positioning a box with
respect to a containing block. [D]
If we do not position the outer box:
#outer { color: red }
#inner {
position: absolute;
top: 200px; left: -100px;
height: 130px; width: 130px;
color: blue;
}
the containing block for inner becomes the initial containing block
(in our example). The following illustration shows where the inner box
would end up in this case.
Image illustrating the effects of absolutely positioning a box with
respect to a containing block established by a normally positioned
parent. [D]
Relative and absolute positioning may be used to implement change
bars, as shown in the following example. The following fragment:

I used two red hyphens to serve as a change bar. They
will "float" to the left of the line containing THIS
--
word.

might result in something like:
Image illustrating the use of floats to create a changebar effect.
[D]
First, the paragraph (whose containing block sides are shown in the
illustration) is flowed normally. Then it is offset '10px' from the
left edge of the containing block (thus, a right margin of '10px' has
been reserved in anticipation of the offset). The two hyphens acting
as change bars are taken out of the flow and positioned at the current
line (due to 'top: auto'), '-1em' from the left edge of its containing
block (established by the P in its final position). The result is that
the change bars seem to "float" to the left of the current line.
9.9 Layered presentation
9.9.1 Specifying the stack level: the 'z-index' property
'z-index'
Value: auto | | inherit
Initial: auto
Applies to: positioned elements
Inherited: no
Percentages: N/A
Media: visual
Computed value: as specified
For a positioned box, the 'z-index' property specifies:
1. The stack level of the box in the current stacking context.
2. Whether the box establishes a local stacking context.
Values have the following meanings:
This integer is the stack level of the generated box in the
current stacking context. The box also establishes a local
stacking context in which its stack level is '0'.
auto
The stack level of the generated box in the current stacking
context is the same as its parent's box. The box does not
establish a new local stacking context.
In this section, the expression "in front of" means closer to the user
as the user faces the screen.
In CSS 2.1, each box has a position in three dimensions. In addition
to their horizontal and vertical positions, boxes lie along a "z-axis"
and are formatted one on top of the other. Z-axis positions are
particularly relevant when boxes overlap visually. This section
discusses how boxes may be positioned along the z-axis.
The order in which the rendering tree is painted onto the canvas is
described in terms of stacking contexts. Stacking contexts can contain
further stacking contexts. A stacking context is atomic from the point
of view of its parent stacking context; boxes in other stacking
contexts may not come between any of its boxes.
Each box belongs to one stacking context. Each box in a given stacking
context has an integer stack level, which is its position on the
z-axis relative to other boxes in the same stacking context. Boxes
with greater stack levels are always formatted in front of boxes with
lower stack levels. Boxes may have negative stack levels. Boxes with
the same stack level in a stacking context are stacked back-to-front
according to document tree order.
The root element forms the root stacking context. Other stacking
contexts are generated by any positioned element (including relatively
positioned elements) having a computed value of 'z-index' other than
'auto'. Stacking contexts are not necessarily related to containing
blocks. In future levels of CSS, other properties may introduce
stacking contexts, for example 'opacity' [CSS3COLOR].
Each stacking context consists of the following stacking levels (from
back to front):
1. the background and borders of the element forming the stacking
context.
2. the stacking contexts of descendants with negative stack levels.
3. a stacking level containing in-flow non-inline-level descendants.
4. a stacking level for floats and their contents.
5. a stacking level for in-flow inline-level descendants.
6. a stacking level for positioned descendants with 'z-index: auto',
and any descendant stacking contexts with 'z-index: 0'.
7. the stacking contexts of descendants with positive stack levels.
For a more thorough explanation of the stacking order, please see
Appendix E.
The contents of inline blocks and inline tables are stacked as if they
generated new stacking contexts, except that any elements that
actually create new stacking contexts take part in the parent stacking
context. They are then painted atomically in the inline stacking
level.
In the following example, the stack levels of the boxes (named with
their "id" attributes) are: "text2"=0, "image"=1, "text3"=2, and
"text1"=3. The "text2" stack level is inherited from the root box. The
others are specified with the 'z-index' property.
Z-order positioning

This text will overlay the butterfly image.

This text will be beneath everything.

This text will underlay text1, but overlay the butterfly image

This example demonstrates the notion of transparency. The default
behavior of the background is to allow boxes behind it to be visible.
In the example, each box transparently overlays the boxes below it.
This behavior can be overridden by using one of the existing
background properties.
9.10 Text direction: the 'direction' and 'unicode-bidi' properties
Conforming user agents that do not support bidirectional text may
ignore the 'direction' and 'unicode-bidi' properties described in this
section. This exception includes UAs that render right-to-left
characters simply because a font on the system contains them but do
not support the concept of right-to-left text direction.
The characters in certain scripts are written from right to left. In
some documents, in particular those written with the Arabic or Hebrew
script, and in some mixed-language contexts, text in a single
(visually displayed) block may appear with mixed directionality. This
phenomenon is called bidirectionality, or "bidi" for short.
The Unicode standard ([UNICODE], section 3.11) defines a complex
algorithm for determining the proper directionality of text. The
algorithm consists of an implicit part based on character properties,
as well as explicit controls for embeddings and overrides. CSS 2.1
relies on this algorithm to achieve proper bidirectional rendering.
The 'direction' and 'unicode-bidi' properties allow authors to specify
how the elements and attributes of a document language map to this
algorithm.
User agents that support bidirectional text must apply the Unicode
bidirectional algorithm to every sequence of inline boxes
uninterrupted by a forced line break or block boundary. This sequence
forms the "paragraph" unit in the bidirectional algorithm. The
paragraph embedding level is set according to the value of the
'direction' property of the containing block rather than by the
heuristic given in steps P2 and P3 of the Unicode algorithm.
Because the directionality of a text depends on the structure and
semantics of the document language, these properties should in most
cases be used only by designers of document type descriptions (DTDs),
or authors of special documents. If a default style sheet specifies
these properties, authors and users should not specify rules to
override them.
The HTML 4 specification ([HTML4], section 8.2) defines
bidirectionality behavior for HTML elements. The style sheet rules
that would achieve the bidi behavior specified in [HTML4] are given in
the sample style sheet. The HTML 4 specification also contains more
information on bidirectionality issues.
'direction'
Value: ltr | rtl | inherit
Initial: ltr
Applies to: all elements, but see prose
Inherited: yes
Percentages: N/A
Media: visual
Computed value: as specified
This property specifies the base writing direction of blocks and the
direction of embeddings and overrides (see 'unicode-bidi') for the
Unicode bidirectional algorithm. In addition, it specifies the
direction of table column layout, the direction of horizontal
overflow, and the position of an incomplete last line in a block in
case of 'text-align: justify'.
Values for this property have the following meanings:
ltr
Left-to-right direction.
rtl
Right-to-left direction.
For the 'direction' property to affect reordering in inline-level
elements, the 'unicode-bidi' property's value must be 'embed' or
'override'.
Note. The 'direction' property, when specified for table column
elements, is not inherited by cells in the column since columns are
not the ancestors of the cells in the document tree. Thus, CSS cannot
easily capture the "dir" attribute inheritance rules described in
[HTML4], section 11.3.2.1.
'unicode-bidi'
Value: normal | embed | bidi-override | inherit
Initial: normal
Applies to: all elements, but see prose
Inherited: no
Percentages: N/A
Media: visual
Computed value: as specified
Values for this property have the following meanings:
normal
The element does not open an additional level of embedding with
respect to the bidirectional algorithm. For inline-level
elements, implicit reordering works across element boundaries.
embed
If the element is inline-level, this value opens an additional
level of embedding with respect to the bidirectional algorithm.
The direction of this embedding level is given by the
'direction' property. Inside the element, reordering is done
implicitly. This corresponds to adding a LRE (U+202A; for
'direction: ltr') or RLE (U+202B; for 'direction: rtl') at the
start of the element and a PDF (U+202C) at the end of the
element.
bidi-override
For inline-level elements this creates an override. For
block-level, table-cell, table-caption, or inline-block
elements this creates an override for inline-level descendents
not within another block-level, table-cell, table-caption, or
inline-block element. This means that inside the element,
reordering is strictly in sequence according to the 'direction'
property; the implicit part of the bidirectional algorithm is
ignored. This corresponds to adding a LRO (U+202D; for
'direction: ltr') or RLO (U+202E; for 'direction: rtl') at the
start of the element or at the start of each anonymous child
block box, if any, and a PDF (U+202C) at the end of the
element.
The final order of characters in each block-level element is the same
as if the bidi control codes had been added as described above, markup
had been stripped, and the resulting character sequence had been
passed to an implementation of the Unicode bidirectional algorithm for
plain text that produced the same line-breaks as the styled text. In
this process, non-textual entities such as images are treated as
neutral characters, unless their 'unicode-bidi' property has a value
other than 'normal', in which case they are treated as strong
characters in the 'direction' specified for the element.
Please note that in order to be able to flow inline boxes in a uniform
direction (either entirely left-to-right or entirely right-to-left),
more inline boxes (including anonymous inline boxes) may have to be
created, and some inline boxes may have to be split up and reordered
before flowing.
Because the Unicode algorithm has a limit of 61 levels of embedding,
care should be taken not to use 'unicode-bidi' with a value other than
'normal' unless appropriate. In particular, a value of 'inherit'
should be used with extreme caution. However, for elements that are,
in general, intended to be displayed as blocks, a setting of
'unicode-bidi: embed' is preferred to keep the element together in
case display is changed to inline (see example below).
The following example shows an XML document with bidirectional text.
It illustrates an important design principle: DTD designers should
take bidi into account both in the language proper (elements and
attributes) and in any accompanying style sheets. The style sheets
should be designed so that bidi rules are separate from other style
rules. The bidi rules should not be overridden by other style sheets
so that the document language's or DTD's bidi behavior is preserved.
Example(s):
In this example, lowercase letters stand for inherently left-to-right
characters and uppercase letters represent inherently right-to-left
characters:
HEBREW1 HEBREW2 english3 HEBREW4 HEBREW5HEBREW6 HEBREW7 HEBREW8english9 english10 english11 HEBREW12 HEBREW13english14 english15 english16english17 HEBREW18 english19 HEBREW20
Since this is XML, the style sheet is responsible for setting the
writing direction. This is the style sheet:
/* Rules for bidi */
HEBREW, HE-QUO {direction: rtl; unicode-bidi: embed}
ENGLISH {direction: ltr; unicode-bidi: embed}
/* Rules for presentation */
HEBREW, ENGLISH, PAR {display: block}
EMPH {font-weight: bold}
The HEBREW element is a block with a right-to-left base direction, the
ENGLISH element is a block with a left-to-right base direction. The
PARs are blocks that inherit the base direction from their parents.
Thus, the first two PARs are read starting at the top right, the final
three are read starting at the top left. Please note that HEBREW and
ENGLISH are chosen as element names for explicitness only; in general,
element names should convey structure without reference to language.
The EMPH element is inline-level, and since its value for
'unicode-bidi' is 'normal' (the initial value), it has no effect on
the ordering of the text. The HE-QUO element, on the other hand,
creates an embedding.
The formatting of this text might look like this if the line length is
long:
5WERBEH 4WERBEH english3 2WERBEH 1WERBEH
8WERBEH 7WERBEH 6WERBEH
english9 english10 english11 13WERBEH 12WERBEH
english14 english15 english16
english17 20WERBEH english19 18WERBEH
Note that the HE-QUO embedding causes HEBREW18 to be to the right of
english19.
If lines have to be broken, it might be more like this:
2WERBEH 1WERBEH
-EH 4WERBEH english3
5WERB
-EH 7WERBEH 6WERBEH
8WERB
english9 english10 en-
glish11 12WERBEH
13WERBEH
english14 english15
english16
english17 18WERBEH
20WERBEH english19
Because HEBREW18 must be read before english19, it is on the line
above english19. Just breaking the long line from the earlier
formatting would not have worked. Note also that the first syllable
from english19 might have fit on the previous line, but hyphenation of
left-to-right words in a right-to-left context, and vice versa, is
usually suppressed to avoid having to display a hyphen in the middle
of a line.
_________________________________________________________________
_________________________________________________________________
10 Visual formatting model details
Contents
* 10.1 Definition of "containing block"
* 10.2 Content width: the 'width' property
* 10.3 Calculating widths and margins
+ 10.3.1 Inline, non-replaced elements
+ 10.3.2 Inline, replaced elements
+ 10.3.3 Block-level, non-replaced elements in normal flow
+ 10.3.4 Block-level, replaced elements in normal flow
+ 10.3.5 Floating, non-replaced elements
+ 10.3.6 Floating, replaced elements
+ 10.3.7 Absolutely positioned, non-replaced elements
+ 10.3.8 Absolutely positioned, replaced elements
+ 10.3.9 'Inline-block', non-replaced elements in normal flow
+ 10.3.10 'Inline-block', replaced elements in normal flow
* 10.4 Minimum and maximum widths: 'min-width' and 'max-width'
* 10.5 Content height: the 'height' property
* 10.6 Calculating heights and margins
+ 10.6.1 Inline, non-replaced elements
+ 10.6.2 Inline replaced elements, block-level replaced
elements in normal flow, 'inline-block' replaced elements in
normal flow and floating replaced elements
+ 10.6.3 Block-level non-replaced elements in normal flow when
'overflow' computes to 'visible'
+ 10.6.4 Absolutely positioned, non-replaced elements
+ 10.6.5 Absolutely positioned, replaced elements
+ 10.6.6 Complicated cases
+ 10.6.7 'Auto' heights for block formatting context roots
* 10.7 Minimum and maximum heights: 'min-height' and 'max-height'
* 10.8 Line height calculations: the 'line-height' and
'vertical-align' properties
+ 10.8.1 Leading and half-leading
10.1 Definition of "containing block"
The position and size of an element's box(es) are sometimes calculated
relative to a certain rectangle, called the containing block of the
element. The containing block of an element is defined as follows:
1. The containing block in which the root element lives is a
rectangle with the dimensions of the viewport, anchored at the
canvas origin for continuous media, and the page area for paged
media. This containing block is called the initial containing
block. The 'direction' property of the initial containing block is
the same as for the root element.
2. For other elements, if the element's position is 'relative' or
'static', the containing block is formed by the content edge of
the nearest block-level, table cell or inline-block ancestor box.
3. If the element has 'position: fixed', the containing block is
established by the viewport in the case of continuous media or the
page area in the case of paged media.
4. If the element has 'position: absolute', the containing block is
established by the nearest ancestor with a 'position' of
'absolute', 'relative' or 'fixed', in the following way:
1. In the case that the ancestor is inline-level, the containing
block depends on the 'direction' property of the ancestor:
1. If the 'direction' is 'ltr', the top and left of the
containing block are the top and left padding edges of
the first box generated by the ancestor, and the bottom
and right are the bottom and right padding edges of the
last box of the ancestor.
2. If the 'direction' is 'rtl', the top and right are the
top and right padding edges of the first box generated
by the ancestor, and the bottom and left are the bottom
and left padding edges of the last box of the ancestor.
Note: This may cause the containing block's width to be
negative.
2. Otherwise, the containing block is formed by the padding edge
of the ancestor.
If there is no such ancestor, the containing block is the initial
containing block.
In paged media, an absolutely positioned element is positioned
relative to its containing block ignoring any page breaks (as if the
document were continuous). The element may subsequently be broken over
several pages.
For absolutely positioned content that resolves to a position on a
page other than the page being laid out (the current page), or
resolves to a position on the current page which has already been
rendered for printing, printers may place the content
* on another location on the current page,
* on a subsequent page, or
* may omit it.
Note that a block-level element that is split over several pages may
have a different width on each page and that there may be
device-specific limits.
Example(s):
With no positioning, the containing blocks (C.B.) in the following
document:
Illustration of containing blocks

This is text in the first paragraph...

This is text in the
second paragraph.

are established as follows:
For box generated by C.B. is established by
html initial C.B. (UA-dependent)
body html
div1 body
p1 div1
p2 div1
em1 p2
strong1 p2
If we position "div1":
#div1 { position: absolute; left: 50px; top: 50px }
its containing block is no longer "body"; it becomes the initial
containing block (since there are no other positioned ancestor boxes).
If we position "em1" as well:
#div1 { position: absolute; left: 50px; top: 50px }
#em1 { position: absolute; left: 100px; top: 100px }
the table of containing blocks becomes:
For box generated by C.B. is established by
html initial C.B. (UA-dependent)
body html
div1 initial C.B.
p1 div1
p2 div1
em1 div1
strong1 em1
By positioning "em1", its containing block becomes the nearest
positioned ancestor box (i.e., that generated by "div1").
10.2 Content width: the 'width' property
'width'
Value: | | auto | inherit
Initial: auto
Applies to: all elements but non-replaced inline elements, table
rows, and row groups
Inherited: no
Percentages: refer to width of containing block
Media: visual
Computed value: the percentage or 'auto' as specified or the
absolute length; 'auto' if the property does not apply
This property specifies the content width of boxes generated by
block-level and replaced elements.
This property does not apply to non-replaced inline-level elements.
The content width of a non-replaced inline element's boxes is that of
the rendered content within them (before any relative offset of
children). Recall that inline boxes flow into line boxes. The width of
line boxes is given by the their containing block, but may be shorted
by the presence of floats.
The width of a replaced element's box is intrinsic and may be scaled
by the user agent if the value of this property is different than
'auto'.
Values have the following meanings:
Specifies the width of the content area using a length unit.
Specifies a percentage width. The percentage is calculated with
respect to the width of the generated box's containing block.
If the containing block's width depends on this element's
width, then the resulting layout is undefined in CSS 2.1. Note:
For absolutely positioned elements whose containing block is
based on a block-level element, the percentage is calculated
with respect to the width of the padding box of that element.
This is a change from CSS1, where the percentage width was
always calculated with respect to the content box of the parent
element.
auto
The width depends on the values of other properties. See the
sections below.
Negative values for 'width' are illegal.
Example(s):
For example, the following rule fixes the content width of paragraphs
at 100 pixels:
p { width: 100px }
10.3 Calculating widths and margins
The values of an element's 'width', 'margin-left', 'margin-right',
'left' and 'right' properties as used for layout depend on the type of
box generated and on each other. (The value used for layout is
sometimes referred to as the used value.) In principle, the values
used are the same as the computed values, with 'auto' replaced by some
suitable value, and percentages calculated based on the containing
block, but there are exceptions. The following situations need to be
distinguished:
1. inline, non-replaced elements
2. inline, replaced elements
3. block-level, non-replaced elements in normal flow
4. block-level, replaced elements in normal flow
5. floating, non-replaced elements
6. floating, replaced elements
7. absolutely positioned, non-replaced elements
8. absolutely positioned, replaced elements
9. 'inline-block', non-replaced elements in normal flow
10. 'inline-block', replaced elements in normal flow
For Points 1-6 and 9-10, the values of 'left' and 'right' used for
layout are determined by the rules in section 9.4.3.
10.3.1 Inline, non-replaced elements
The 'width' property does not apply. A computed value of 'auto' for
'left', 'right', 'margin-left' or 'margin-right' becomes a used value
of '0'.
10.3.2 Inline, replaced elements
A computed value of 'auto' for 'margin-left' or 'margin-right' becomes
a used value of '0'.
If 'height' and 'width' both have computed values of 'auto' and the
element also has an intrinsic width, then that intrinsic width is the
used value of 'width'.
If 'height' and 'width' both have computed values of 'auto' and the
element has no intrinsic width, but does have an intrinsic height and
intrinsic ratio; or if 'width' has a computed value of 'auto',
'height' has some other computed value, and the element does have an
intrinsic ratio; then the used value of 'width' is:
(used height) * (intrinsic ratio)
If 'height' and 'width' both have computed values of 'auto' and the
element has an intrinsic ratio but no intrinsic height or width and
the containing block's width doesn't itself depend on the replaced
element's width, then the used value of 'width' is calculated from the
constraint equation used for block-level, non-replaced elements in
normal flow.
Otherwise, if 'width' has a computed value of 'auto', but none of the
conditions above are met, then the used value of 'width' becomes
300px. If 300px is too wide to fit the device, UAs should use the
width of the largest rectangle that has a 2:1 ratio and fits the
device instead.
Percentage intrinsic widths are first evaluated with respect to the
containing block's width, if that width doesn't itself depend on the
replaced element's width. If it does, then a percentage intrinsic
width on that element can't be resolved and the element is assumed to
have no intrinsic width.
10.3.3 Block-level, non-replaced elements in normal flow
The following constraints must hold among the used values of the other
properties:
'margin-left' + 'border-left-width' + 'padding-left' + 'width' +
'padding-right' + 'border-right-width' + 'margin-right' + scrollbar
width (if any) = width of containing block
If 'width' is not 'auto' and 'border-left-width' + 'padding-left' +
'width' + 'padding-right' + 'border-right-width' + scrollbar width (if
any) (plus any of 'margin-left' or 'margin-right' that are not 'auto')
is larger than the width of the containing block, then any 'auto'
values for 'margin-left' or 'margin-right' are, for the following
rules, treated as zero.
If all of the above have a computed value other than 'auto', the
values are said to be "over-constrained" and one of the used values
will have to be different from its computed value. If the 'direction'
property of the containing block has the value 'ltr', the specified
value of 'margin-right' is ignored and the value is calculated so as
to make the equality true. If the value of 'direction' is 'rtl', this
happens to 'margin-left' instead.
If there is exactly one value specified as 'auto', its used value
follows from the equality.
If 'width' is set to 'auto', any other 'auto' values become '0' and
'width' follows from the resulting equality.
If both 'margin-left' and 'margin-right' are 'auto', their used values
are equal. This horizontally centers the element with respect to the
edges of the containing block.
The "scrollbar width" value is only relevant if the user agent uses a
scrollbar as its scrolling mechanism. See the definition of the
'overflow' property.
10.3.4 Block-level, replaced elements in normal flow
The used value of 'width' is determined as for inline replaced
elements. Then the rules for non-replaced block-level elements are
applied to determine the margins.
10.3.5 Floating, non-replaced elements
If 'margin-left', or 'margin-right' are computed as 'auto', their used
value is '0'.
If 'width' is computed as 'auto', the used value is the
"shrink-to-fit" width.
Calculation of the shrink-to-fit width is similar to calculating the
width of a table cell using the automatic table layout algorithm.
Roughly: calculate the preferred width by formatting the content
without breaking lines other than where explicit line breaks occur,
and also calculate the preferred minimum width, e.g., by trying all
possible line breaks. CSS 2.1 does not define the exact algorithm.
Thirdly, find the available width: in this case, this is the width of
the containing block minus the used values of 'margin-left',
'border-left-width', 'padding-left', 'padding-right',
'border-right-width', 'margin-right', and the widths of any relevant
scroll bars.
Then the shrink-to-fit width is: min(max(preferred minimum width,
available width), preferred width).
10.3.6 Floating, replaced elements
If 'margin-left' or 'margin-right' are computed as 'auto', their used
value is '0'. The used value of 'width' is determined as for inline
replaced elements.
10.3.7 Absolutely positioned, non-replaced elements
For the purposes of this section and the next, the term "static
position" (of an element) refers, roughly, to the position an element
would have had in the normal flow. More precisely:
* The static position for 'left' is the distance from the left edge
of the containing block to the left margin edge of a hypothetical
box that would have been the first box of the element if its
'position' property had been 'static' and 'float' had been 'none'.
The value is negative if the hypothetical box is to the left of
the containing block.
* The static position for 'right' is the distance from the right
edge of the containing block to the right margin edge of the same
hypothetical box as above. The value is positive if the
hypothetical box is to the left of the containing block's edge.
But rather than actually calculating the dimensions of that
hypothetical box, user agents are free to make a guess at its probable
position.
For the purposes of calculating the static position, the containing
block of fixed positioned elements is the initial containing block
instead of the viewport, and all scrollable boxes should be assumed to
be scrolled to their origin.
The constraint that determines the used values for these elements is:
'left' + 'margin-left' + 'border-left-width' + 'padding-left' +
'width' + 'padding-right' + 'border-right-width' + 'margin-right' +
'right' + scrollbar width (if any) = width of containing block
If all three of 'left', 'width', and 'right' are 'auto': First set any
'auto' values for 'margin-left' and 'margin-right' to 0. Then, if the
'direction' property of the containing block is 'ltr' set 'left' to
the static position and apply rule number three below; otherwise, set
'right' to the static position and apply rule number one below.
If none of the three is 'auto': If both 'margin-left' and
'margin-right' are 'auto', solve the equation under the extra
constraint that the two margins get equal values, unless this would
make them negative, in which case when direction of the containing
block is 'ltr' ('rtl'), set 'margin-left' ('margin-right') to zero and
solve for 'margin-right' ('margin-left'). If one of 'margin-left' or
'margin-right' is 'auto', solve the equation for that value. If the
values are over-constrained, ignore the value for 'left' (in case the
'direction' property of the containing block is 'rtl') or 'right' (in
case 'direction' is 'ltr') and solve for that value.
Otherwise, set 'auto' values for 'margin-left' and 'margin-right' to
0, and pick the one of the following six rules that applies.
1. 'left' and 'width' are 'auto' and 'right' is not 'auto', then the
width is shrink-to-fit. Then solve for 'left'
2. 'left' and 'right' are 'auto' and 'width' is not 'auto', then if
the 'direction' property of the containing block is 'ltr' set
'left' to the static position, otherwise set 'right' to the static
position. Then solve for 'left' (if 'direction is 'rtl') or
'right' (if 'direction' is 'ltr').
3. 'width' and 'right' are 'auto' and 'left' is not 'auto', then the
width is shrink-to-fit . Then solve for 'right'
4. 'left' is 'auto', 'width' and 'right' are not 'auto', then solve
for 'left'
5. 'width' is 'auto', 'left' and 'right' are not 'auto', then solve
for 'width'
6. 'right' is 'auto', 'left' and 'width' are not 'auto', then solve
for 'right'
Calculation of the shrink-to-fit width is similar to calculating the
width of a table cell using the automatic table layout algorithm.
Roughly: calculate the preferred width by formatting the content
without breaking lines other than where explicit line breaks occur,
and also calculate the preferred minimum width, e.g., by trying all
possible line breaks. CSS 2.1 does not define the exact algorithm.
Thirdly, calculate the available width: this is found by solving for
'width' after setting 'left' (in case 1) or 'right' (in case 3) to 0.
Then the shrink-to-fit width is: min(max(preferred minimum width,
available width), preferred width).
The "scrollbar width" value is only relevant if the user agent uses a
scrollbar as its scrolling mechanism. See the definition of the
'overflow' property.
10.3.8 Absolutely positioned, replaced elements
This situation is similar to the previous one, except that the element
has an intrinsic width. The sequence of substitutions is now:
1. The used value of 'width' is determined as for inline replaced
elements.
2. If both 'left' and 'right' have the value 'auto', then if
'direction' of the containing block is 'ltr', set 'left' to the
static position; else if 'direction' is 'rtl', set 'right' to the
static position.
3. If 'left' or 'right' are 'auto', replace any 'auto' on
'margin-left' or 'margin-right' with '0'.
4. If at this point both 'margin-left' and 'margin-right' are still
'auto', solve the equation under the extra constraint that the two
margins must get equal values, unless this would make them
negative, in which case when the direction of the containing block
is 'ltr' ('rtl'), set 'margin-left' ('margin-right') to zero and
solve for 'margin-right' ('margin-left').
5. If at this point there is an 'auto' left, solve the equation for
that value.
6. If at this point the values are over-constrained, ignore the value
for either 'left' (in case the 'direction' property of the
containing block is 'rtl') or 'right' (in case 'direction' is
'ltr') and solve for that value.
10.3.9 'Inline-block', non-replaced elements in normal flow
If 'width' is 'auto', the used value is the shrink-to-fit width as for
floating elements.
A computed value of 'auto' for 'margin-left' or 'margin-right' becomes
a used value of '0'.
10.3.10 'Inline-block', replaced elements in normal flow
Exactly as inline replaced elements.
10.4 Minimum and maximum widths: 'min-width' and 'max-width'
'min-width'
Value: | | inherit
Initial: 0
Applies to: all elements but non-replaced inline elements, table
rows, and row groups
Inherited: no
Percentages: refer to width of containing block
Media: visual
Computed value: the percentage as specified or the absolute length
'max-width'
Value: | | none | inherit
Initial: none
Applies to: all elements but non-replaced inline elements, table
rows, and row groups
Inherited: no
Percentages: refer to width of containing block
Media: visual
Computed value: the percentage as specified or the absolute length
or 'none'
These two properties allow authors to constrain content widths to a
certain range. Values have the following meanings:
Specifies a fixed minimum or maximum used width.
Specifies a percentage for determining the used value. The
percentage is calculated with respect to the width of the
generated box's containing block. If the containing block's
width is negative, the used value is zero. If the containing
block's width depends on this element's width, then the
resulting layout is undefined in CSS 2.1.
none
(Only on 'max-width') No limit on the width of the box.
Negative values for 'min-width' and 'max-width' are illegal.
The following algorithm describes how the two properties influence the
used value of the 'width' property:
1. The tentative used width is calculated (without 'min-width' and
'max-width') following the rules under "Calculating widths and
margins" above.
2. If the tentative used width is greater than 'max-width', the rules
above are applied again, but this time using the computed value of
'max-width' as the computed value for 'width'.
3. If the resulting width is smaller than 'min-width', the rules
above are applied again, but this time using the value of
'min-width' as the computed value for 'width'.
These steps do not affect the real computed values of the above
properties.
However, for replaced elements with an intrinsic ratio and both
'width' and 'height' specified as 'auto', the algorithm is as follows:
Select from the table the resolved height and width values for the
appropriate constraint violation. Take the max-width and max-height as
max(min, max) so that min =< max holds true. In this table w and h
stand for the results of the width and height computations ignoring
the 'min-width', 'min-height', 'max-width' and 'max-height'
properties. Normally these are the intrinsic width and height, but
they may not be in the case of replaced elements with intrinsic
ratios.
Note: In cases where an explicit width or height is set and the other
dimension is auto, applying a minimum or maximum constraint on the
auto side can cause an over-constrained situation. The spec is clear
in the behavior but it might not be what the author expects. The CSS3
image-fit property can be used to obtain different results in this
situation.
Constraint Violation Resolved Width Resolved Height
none w h
w > max-width max-width max(max-width * h/w, min-height)
w < min-width min-width min(min-width * h/w, max-height)
h > max-height max(max-height * w/h, min-width) max-height
h < min-height min(min-height * w/h, max-width) min-height
(w > max-width) and (h > max-height), where
(max-width/w =< max-height/h) max-width max(min-height,
max-width * h/w)
(w > max-width) and (h > max-height), where
(max-width/w > max-height/h) max(min-width, max-height * w/h)
max-height
(w < min-width) and (h < min-height), where
(min-width/w =< min-height/h) min(max-width, min-height * w/h)
min-height
(w < min-width) and (h < min-height), where
(min-width/w > min-height/h) min-width min(max-height,
min-width * h/w)
(w < min-width) and (h > max-height) min-width max-height
(w > max-width) and (h < min-height) max-width min-height
Then apply the rules under "Calculating widths and margins" above, as
if 'width' were computed as this value.
10.5 Content height: the 'height' property
'height'
Value: | | auto | inherit
Initial: auto
Applies to: all elements but non-replaced inline elements, table
columns, and column groups
Inherited: no
Percentages: see prose
Media: visual
Computed value: the percentage or 'auto' (see prose under
) or the absolute length; 'auto' if the property does not
apply
This property specifies the content height of boxes generated by
block-level, inline-block and replaced elements.
This property does not apply to non-replaced inline-level elements.
See the section on computing heights and margins for non-replaced
inline elements for the rules used instead.
Values have the following meanings:
Specifies the height of the content area using a length value.
Specifies a percentage height. The percentage is calculated
with respect to the height of the generated box's containing
block. If the height of the containing block is not specified
explicitly (i.e., it depends on content height), and this
element is not absolutely positioned, the value computes to
'auto'. A percentage height on the root element is relative to
the initial containing block.
auto
The height depends on the values of other properties. See the
prose below.
Note that the height of the containing block of an absolutely
positioned element is independent of the size of the element itself,
and thus a percentage height on such an element can always be
resolved. However, it may be that the height is not known until
elements that come later in the document have been processed.
Negative values for 'height' are illegal.
Example(s):
For example, the following rule sets the content height of paragraphs
to 100 pixels:
p { height: 100px }
Paragraphs of which the height of the contents exceeds 100 pixels will
overflow according to the 'overflow' property.
10.6 Calculating heights and margins
For calculating the values of 'top', 'margin-top', 'height',
'margin-bottom', and 'bottom' a distinction must be made between
various kinds of boxes:
1. inline, non-replaced elements
2. inline, replaced elements
3. block-level, non-replaced elements in normal flow
4. block-level, replaced elements in normal flow
5. floating, non-replaced elements
6. floating, replaced elements
7. absolutely positioned, non-replaced elements
8. absolutely positioned, replaced elements
9. 'inline-block', non-replaced elements in normal flow
10. 'inline-block', replaced elements in normal flow
For Points 1-6 and 9-10, the used values of 'top' and 'bottom' are
determined by the rules in section 9.4.3.
Note: these rules apply to the root element just as to any other
element.
10.6.1 Inline, non-replaced elements
The 'height' property doesn't apply. The height of the content area
should be based on the font, but this specification does not specify
how. A UA may, e.g., use the em-box or the maximum ascender and
descender of the font. (The latter would ensure that glyphs with parts
above or below the em-box still fall within the content area, but
leads to differently sized boxes for different fonts; the former would
ensure authors can control background styling relative to the
'line-height', but leads to glyphs painting outside their content
area.)
Note: level 3 of CSS will probably include a property to select which
measure of the font is used for the content height.
The vertical padding, border and margin of an inline, non-replaced box
start at the top and bottom of the content area, not the
'line-height'. But only the 'line-height' is used when calculating the
height of the line box.
If more than one font is used (this could happen when glyphs are found
in different fonts), the height of the content area is not defined by
this specification. However, we suggest that the height is chosen such
that the content area is just high enough for either (1) the em-boxes,
or (2) the maximum ascenders and descenders, of all the fonts in the
element. Note that this may be larger than any of the font sizes
involved, depending on the baseline alignment of the fonts.
10.6.2 Inline replaced elements, block-level replaced elements in normal
flow, 'inline-block' replaced elements in normal flow and floating replaced
elements
If 'margin-top', or 'margin-bottom' are 'auto', their used value is 0.
If 'height' and 'width' both have computed values of 'auto' and the
element also has an intrinsic height, then that intrinsic height is
the used value of 'height'.
Otherwise, if 'height' has a computed value of 'auto', and the element
has an intrinsic ratio then the used value of 'height' is:
(used width) / (intrinsic ratio)
Otherwise, if 'height' has a computed value of 'auto', but none of the
conditions above are met, then the used value of 'height' must be set
to the height of the largest rectangle that has a 2:1 ratio, has a
height not greater than 150px, and has a width not greater than the
device width.
Percentage intrinsic heights are evaluated with respect to the
containing block's height, if that height is specified explicitly, or
if the replaced element is absolutely positioned. If neither of these
conditions is met, then percentage values on such replaced elements
can't be resolved and such elements are assumed to have no intrinsic
height.
For 'inline' and 'inline-block' elements, the margin box is used when
calculating the height of the line box.
10.6.3 Block-level non-replaced elements in normal flow when 'overflow'
computes to 'visible'
This section also applies to block-level non-replaced elements in
normal flow when 'overflow' does not compute to 'visible' but has been
propagated to the viewport.
If 'margin-top', or 'margin-bottom' are 'auto', their used value is 0.
If 'height' is 'auto', the height depends on whether the element has
any block-level children and whether it has padding or borders:
If it only has inline-level children, the height is the distance
between the top of the topmost line box and the bottom of the
bottommost line box.
If it has block-level children, the height is the distance between the
top border-edge of the topmost block-level child box that doesn't have
margins collapsed through it and the bottom border-edge of the
bottommost block-level child box that doesn't have margins collapsed
through it. However, if the element has a non-zero top padding and/or
top border, or is the root element, then the content starts at the top
margin edge of the topmost child. (The first case expresses the fact
that the top and bottom margins of the element collapse with those of
the topmost and bottommost children, while in the second case the
presence of the padding/border prevents the top margins from
collapsing.) Similarly, if the bottom margin of the block does not
collapse with the bottom margin of its last in-flow child, then the
content ends at the bottom margin edge of the bottommost child.
Only children in the normal flow are taken into account (i.e.,
floating boxes and absolutely positioned boxes are ignored, and
relatively positioned boxes are considered without their offset). Note
that the child box may be an anonymous block box.
10.6.4 Absolutely positioned, non-replaced elements
For the purposes of this section and the next, the term "static
position" (of an element) refers, roughly, to the position an element
would have had in the normal flow. More precisely, the static position
for 'top' is the distance from the top edge of the containing block to
the top margin edge of a hypothetical box that would have been the
first box of the element if its 'position' property had been 'static'
and 'float' had been 'none'. The value is negative if the hypothetical
box is above the containing block.
But rather than actually calculating the dimensions of that
hypothetical box, user agents are free to make a guess at its probable
position.
For the purposes of calculating the static position, the containing
block of fixed positioned elements is the initial containing block
instead of the viewport.
For absolutely positioned elements, the used values of the vertical
dimensions must satisfy this constraint:
'top' + 'margin-top' + 'border-top-width' + 'padding-top' +
'height' + 'padding-bottom' + 'border-bottom-width' +
'margin-bottom' + 'bottom' + scrollbar height (if any) = height of
containing block
If all three of 'top', 'height', and 'bottom' are auto, set 'top' to
the static position and apply rule number three below.
If none of the three are 'auto': If both 'margin-top' and
'margin-bottom' are 'auto', solve the equation under the extra
constraint that the two margins get equal values. If one of
'margin-top' or 'margin-bottom' is 'auto', solve the equation for that
value. If the values are over-constrained, ignore the value for
'bottom' and solve for that value.
Otherwise, pick the one of the following six rules that applies.
1. 'top' and 'height' are 'auto' and 'bottom' is not 'auto', then the
height is based on the content, set 'auto' values for 'margin-top'
and 'margin-bottom' to 0, and solve for 'top'
2. 'top' and 'bottom' are 'auto' and 'height' is not 'auto', then set
'top' to the static position, set 'auto' values for 'margin-top'
and 'margin-bottom' to 0, and solve for 'bottom'
3. 'height' and 'bottom' are 'auto' and 'top' is not 'auto', then the
height is based on the content, set 'auto' values for 'margin-top'
and 'margin-bottom' to 0, and solve for 'bottom'
4. 'top' is 'auto', 'height' and 'bottom' are not 'auto', then set
'auto' values for 'margin-top' and 'margin-bottom' to 0, and solve
for 'top'
5. 'height' is 'auto', 'top' and 'bottom' are not 'auto', then 'auto'
values for 'margin-top' and 'margin-bottom' are set to 0 and solve
for 'height'
6. 'bottom' is 'auto', 'top' and 'height' are not 'auto', then set
'auto' values for 'margin-top' and 'margin-bottom' to 0 and solve
for 'bottom'
The "scrollbar height" value is only relevant if the user agent uses a
scrollbar as its scrolling mechanism. See the definition of the
'overflow' property.
10.6.5 Absolutely positioned, replaced elements
This situation is similar to the previous one, except that the element
has an intrinsic height. The sequence of substitutions is now:
1. The used value of 'height' is determined as for inline replaced
elements.
2. If both 'top' and 'bottom' have the value 'auto', replace 'top'
with the element's static position.
3. If 'bottom' is 'auto', replace any 'auto' on 'margin-top' or
'margin-bottom' with '0'.
4. If at this point both 'margin-top' and 'margin-bottom' are still
'auto', solve the equation under the extra constraint that the two
margins must get equal values.
5. If at this point there is only one 'auto' left, solve the equation
for that value.
6. If at this point the values are over-constrained, ignore the value
for 'bottom' and solve for that value.
10.6.6 Complicated cases
This section applies to:
* Block-level, non-replaced elements in normal flow when 'overflow'
does not compute to 'visible' (except if the 'overflow' property's
value has been propagated to the viewport).
* 'Inline-block', non-replaced elements.
* Floating, non-replaced elements.
If 'margin-top', or 'margin-bottom' are 'auto', their used value is 0.
If 'height' is 'auto', the height depends on the element's
descendants.
For 'inline-block' elements, the margin box is used when calculating
the height of the line box.
10.6.7 'Auto' heights for block formatting context roots
In certain cases (see the preceding sections), the height of an
element is computed as follows:
If it only has inline-level children, the height is the distance
between the top of the topmost line box and the bottom of the
bottommost line box.
If it has block-level children, the height is the distance between the
top margin-edge of the topmost block-level child box and the bottom
margin-edge of the bottommost block-level child box.
Absolutely positioned children are ignored, and relatively positioned
boxes are considered without their offset. Note that the child box may
be an anonymous block box.
In addition, if the element has any floating descendants whose bottom
margin edge is below the bottom, then the height is increased to
include those edges. Only floats that are children of the element
itself or of descendants in the normal flow are taken into account,
e.g., floats inside absolutely positioned descendants or other floats
are not.
10.7 Minimum and maximum heights: 'min-height' and 'max-height'
It is sometimes useful to constrain the height of elements to a
certain range. Two properties offer this functionality:
'min-height'
Value: | | inherit
Initial: 0
Applies to: all elements but non-replaced inline elements, table
columns, and column groups
Inherited: no
Percentages: see prose
Media: visual
Computed value: the percentage as specified or the absolute length
'max-height'
Value: | | none | inherit
Initial: none
Applies to: all elements but non-replaced inline elements, table
columns, and column groups
Inherited: no
Percentages: see prose
Media: visual
Computed value: the percentage as specified or the absolute length
or 'none'
These two properties allow authors to constrain box heights to a
certain range. Values have the following meanings:
Specifies a fixed minimum or maximum computed height.
Specifies a percentage for determining the used value. The
percentage is calculated with respect to the height of the
generated box's containing block. If the height of the
containing block is not specified explicitly (i.e., it depends
on content height), and this element is not absolutely
positioned, the percentage value is treated as '0' (for
'min-height') or 'none' (for 'max-height').
none
(Only on 'max-height') No limit on the height of the box.
Negative values for 'min-height' and 'max-height' are illegal.
The following algorithm describes how the two properties influence the
used value of the 'height' property:
1. The tentative used height is calculated (without 'min-height' and
'max-height') following the rules under "Calculating heights and
margins" above.
2. If this tentative height is greater than 'max-height', the rules
above are applied again, but this time using the value of
'max-height' as the computed value for 'height'.
3. If the resulting height is smaller than 'min-height', the rules
above are applied again, but this time using the value of
'min-height' as the computed value for 'height'.
These steps do not affect the real computed values of the above
properties.
However, for replaced elements with both 'width' and 'height' computed
as 'auto', use the algorithm under Minimum and maximum widths above to
find the used width and height. Then apply the rules under "Computing
heights and margins" above, using the resulting width and height as if
they were the computed values.
10.8 Line height calculations: the 'line-height' and 'vertical-align'
properties
As described in the section on inline formatting contexts, user agents
flow inline boxes into a vertical stack of line boxes. The height of a
line box is determined as follows:
1. The height of each inline box in the line box is calculated (see
"Calculating heights and margins" and the 'line-height' property).
2. The inline boxes are aligned vertically according to their
'vertical-align' property.
3. The line box height is the distance between the uppermost box top
and the lowermost box bottom.
4. If the resulting height is smaller than the minimal height of line
boxes for this block, as specified by the 'line-height' property,
the height is increased to be that minimal height.
Empty inline elements generate empty inline boxes, but these boxes
still have margins, padding, borders and a line height, and thus
influence these calculations just like elements with content.
10.8.1 Leading and half-leading
Since the value of 'line-height' may be different from the height of
the content area there may be space above and below rendered glyphs.
The difference between the content height and the used value of
'line-height' is called the leading. Half the leading is called the
half-leading.
User agents center glyphs vertically in an inline box, adding
half-leading on the top and bottom. For example, if a piece of text is
'12px' high and the 'line-height' value is '14px', 2pxs of extra space
should be added: 1px above and 1px below the letters. (This applies to
empty boxes as well, as if the empty box contained an infinitely
narrow letter.)
When the 'line-height' value is less than the content height, the
final inline box height will be less than the font size and the
rendered glyphs will "bleed" outside the box. If such a box touches
the edge of a line box, the rendered glyphs will also "bleed" into the
adjoining line box.
Although margins, borders, and padding of non-replaced elements do not
enter into the line box calculation, they are still rendered around
inline boxes. This means that if the height specified by 'line-height'
is less than the content height of contained boxes, backgrounds and
colors of padding and borders may "bleed" into adjoining line boxes.
User agents should render the boxes in document order. This will cause
the borders on subsequent lines to paint over the borders and text of
previous lines.
'line-height'
Value: normal | | | | inherit
Initial: normal
Applies to: all elements
Inherited: yes
Percentages: refer to the font size of the element itself
Media: visual
Computed value: for and the absolute value;
otherwise as specified
On a block-level, table-cell, table-caption or inline-block element
whose content is composed of inline-level elements, 'line-height'
specifies the minimal height of line boxes within the element. The
minimum height consists of a minimum height above the block's baseline
and a minimum depth below it, exactly as if each line box starts with
a zero-width inline box with the block's font and line height
properties (what T[E]X calls a "strut").
On an inline-level element, 'line-height' specifies the height that is
used in the calculation of the line box height (except for inline
replaced elements, where the height of the box is given by the
'height' property).
Values for this property have the following meanings:
normal
Tells user agents to set the used value to a "reasonable" value
based on the font of the element. The value has the same
meaning as . We recommend a used value for 'normal'
between 1.0 to 1.2. The computed value is 'normal'.
The specified length is used in the calculation of the line box
height. Negative values are illegal.
The used value of the property is this number multiplied by the
element's font size. Negative values are illegal. The computed
value is the same as the specified value.
The computed value of the property is this percentage
multiplied by the element's computed font size. Negative values
are illegal.
Example(s):
The three rules in the example below have the same resultant line
height:
div { line-height: 1.2; font-size: 10pt } /* number */
div { line-height: 1.2em; font-size: 10pt } /* length */
div { line-height: 120%; font-size: 10pt } /* percentage */
When an element contains text that is rendered in more than one font,
user agents may determine the 'line-height' value according to the
largest font size.
Generally, when there is only one value of 'line-height' for all
inline boxes in a paragraph (and no tall images), the above will
ensure that baselines of successive lines are exactly 'line-height'
apart. This is important when columns of text in different fonts have
to be aligned, for example in a table.
'vertical-align'
Value: baseline | sub | super | top | text-top | middle | bottom |
text-bottom | | | inherit
Initial: baseline
Applies to: inline-level and 'table-cell' elements
Inherited: no
Percentages: refer to the 'line-height' of the element itself
Media: visual
Computed value: for and the absolute length,
otherwise as specified
This property affects the vertical positioning inside a line box of
the boxes generated by an inline-level element.
Note. Values of this property have slightly different meanings in the
context of tables. Please consult the section on table height
algorithms for details.
The following values only have meaning with respect to a parent
inline-level element, or to the strut of a parent block-level,
table-cell, table-caption or inline-block element.
baseline
Align the baseline of the box with the baseline of the parent
box. If the box doesn't have a baseline, align the bottom
margin edge with the parent's baseline.
middle
Align the vertical midpoint of the box with the baseline of the
parent box plus half the x-height of the parent.
sub
Lower the baseline of the box to the proper position for
subscripts of the parent's box. (This value has no effect on
the font size of the element's text.)
super
Raise the baseline of the box to the proper position for
superscripts of the parent's box. (This value has no effect on
the font size of the element's text.)
text-top
Align the top of the box with the top of the parent's content
area (see 10.6.1).
text-bottom
Align the bottom of the box with the bottom of the parent's
content area (see 10.6.1).
Raise (positive value) or lower (negative value) the box by
this distance (a percentage of the 'line-height' value). The
value '0%' means the same as 'baseline'.
Raise (positive value) or lower (negative value) the box by
this distance. The value '0cm' means the same as 'baseline'.
The following values align the element relative to the line box. Since
the element may have children aligned relative to it (which in turn
may have descendants aligned relative to them), these values use the
bounds of the aligned subtree. The aligned subtree of an inline
element contains that element and the aligned subtrees of all children
inline elements whose computed 'vertical-align' value is not 'top' or
'bottom'. The top of the aligned subtree is the highest of the tops of
the boxes in the subtree, and the bottom is analogous.
top
Align the top of the aligned subtree with the top of the line
box.
bottom
Align the bottom of the aligned subtree with the bottom of the
line box.
The baseline of an 'inline-table' is the baseline of the first row of
the table.
The baseline of an 'inline-block' is the baseline of its last line box
in the normal flow, unless it has either no in-flow line boxes or if
its 'overflow' property has a computed value other than 'visible', in
which case the baseline is the bottom margin edge.
_________________________________________________________________
_________________________________________________________________
11 Visual effects
Contents
* 11.1 Overflow and clipping
+ 11.1.1 Overflow: the 'overflow' property
+ 11.1.2 Clipping: the 'clip' property
* 11.2 Visibility: the 'visibility' property
11.1 Overflow and clipping
Generally, the content of a block box is confined to the content edges
of the box. In certain cases, a box may overflow, meaning its content
lies partly or entirely outside of the box, e.g.:
* A line cannot be broken, causing the line box to be wider than the
block box.
* A block-level box is too wide for the containing block. This may
happen when an element's 'width' property has a value that causes
the generated block box to spill over sides of the containing
block.
* An element's height exceeds an explicit height assigned to the
containing block (i.e., the containing block's height is
determined by the 'height' property, not by content height).
* A descendent box is positioned absolutely, partly outside the box.
Such boxes are not always clipped by the overflow property on
their ancestors.
* A descendent box has negative margins, causing it to be positioned
partly outside the box.
* The 'text-indent' property causes an inline box to hang off either
the left or right edge of the block box.
Whenever overflow occurs, the 'overflow' property specifies whether a
box is clipped to its padding edge, and if so, whether a scrolling
mechanism is provided to access any clipped out content.
11.1.1 Overflow: the 'overflow' property
'overflow'
Value: visible | hidden | scroll | auto | inherit
Initial: visible
Applies to: non-replaced block-level elements, table cells, and
inline-block elements
Inherited: no
Percentages: N/A
Media: visual
Computed value: as specified
This property specifies whether content of a block-level element is
clipped when it overflows the element's box. It affects the clipping
of all of the element's content except any descendant elements (and
their respective content and descendants) whose containing block is
the viewport or an ancestor of the element. Values have the following
meanings:
visible
This value indicates that content is not clipped, i.e., it may
be rendered outside the block box.
hidden
This value indicates that the content is clipped and that no
scrolling user interface should be provided to view the content
outside the clipping region.
scroll
This value indicates that the content is clipped and that if
the user agent uses a scrolling mechanism that is visible on
the screen (such as a scroll bar or a panner), that mechanism
should be displayed for a box whether or not any of its content
is clipped. This avoids any problem with scrollbars appearing
and disappearing in a dynamic environment. When this value is
specified and the target medium is 'print', overflowing content
may be printed.
auto
The behavior of the 'auto' value is user agent-dependent, but
should cause a scrolling mechanism to be provided for
overflowing boxes.
Even if 'overflow' is set to 'visible', content may be clipped to a
UA's document window by the native operating environment.
UAs must apply the 'overflow' property set on the root element to the
viewport. HTML UAs must instead apply the 'overflow' property from the
BODY element to the viewport, if the value on the HTML element is
'visible'. The 'visible' value when used for the viewport must be
interpreted as 'auto'. The element from which the value is propagated
must have a used value for 'overflow' of 'visible'.
In the case of a scrollbar being placed on an edge of the element's
box, it should be inserted between the inner border edge and the outer
padding edge. The space taken up by the scrollbars affects the
computation of the dimensions in the rendering model.
Example(s):
Consider the following example of a block quotation (

)
that is too big for its containing block (established by a

).
Here is the source:

I didn't like the play, but then I saw
it under adverse conditions - the curtain was up.

would be formatted without clipping, something like this:
Rendered overflow [D]
Setting 'overflow' to 'hidden' for the

, on the other hand,
causes the

to be clipped by the containing block:
Clipped overflow [D]
A value of 'scroll' would tell UAs that support a visible scrolling
mechanism to display one so that users could access the clipped
content.
Finally, consider this case where an absolutely positioned element is
mixed with an overflow parent.
Style sheet:
container { position: relative; border: solid; }
scroller { overflow: scroll; height: 5em; margin: 5em; }
satellite { position: absolute; top: 0; }
body { height: 10em; }
Document fragment:
In this example, the "scroller" element will not scroll the
"satellite" element, because the latter's containing block is outside
the element whose overflow is being clipped and scrolled.
11.1.2 Clipping: the 'clip' property
A clipping region defines what portion of an element's border box is
visible. By default, the element is not clipped. However, the clipping
region may be explicitly set with the 'clip' property.
'clip'
Value: | auto | inherit
Initial: auto
Applies to: absolutely positioned elements
Inherited: no
Percentages: N/A
Media: visual
Computed value: For rectangle values, a rectangle consisting of four
computed lengths; otherwise, as specified
The 'clip' property applies only to absolutely positioned elements.
Values have the following meanings:
auto
The element does not clip.
In CSS 2.1, the only valid value is: rect(,
, , ) where and specify
offsets from the top border edge of the box, and , and
specify offsets from the left border edge of the box in
left-to-right text and from the right border edge of the box in
right-to-left text. Authors should separate offset values with
commas. User agents must support separation with commas, but
may also support separation without commas, because a previous
revision of this specification was ambiguous in this respect.
, , , and may either have a
value or 'auto'. Negative lengths are permitted. The value
'auto' means that a given edge of the clipping region will be
the same as the edge of the element's generated border box
(i.e., 'auto' means the same as '0' for and (in
left-to-right text, in right-to-left text), the same as
the computed value of the height plus the sum of vertical
padding and border widths for , and the same as the
computed value of the width plus the sum of the horizontal
padding and border widths for (in left-to-right text,
in right-to-left text), such that four 'auto' values
result in the clipping region being the same as the element's
border box).
When coordinates are rounded to pixel coordinates, care should
be taken that no pixels remain visible when and
have the same value (or and have the same
value), and conversely that no pixels within the element's
border box remain hidden when these values are 'auto'.
An element's clipping region clips out any aspect of the element (e.g.
content, children, background, borders, text decoration, outline and
visible scrolling mechanism -- if any) that is outside the clipping
region. Content that has been clipped does not cause overflow.
The element's ancestors may also clip portions of their content (e.g.
via their own 'clip' property and/or if their 'overflow' property is
not 'visible'); what is rendered is the cumulative intersection.
If the clipping region exceeds the bounds of the UA's document window,
content may be clipped to that window by the native operating
environment.
Example(s):
The following two rules:
p { clip: rect(5px, 40px, 45px, 5px); }
p { clip: rect(5px, 55px, 45px, 5px); }
will create the rectangular clipping regions delimited by the dashed
lines in the following illustrations:
Two clipping regions [D]
Note. In CSS 2.1, all clipping regions are rectangular. We anticipate
future extensions to permit non-rectangular clipping. Future updates
may also reintroduce a syntax for offsetting shapes from each edge
instead of offsetting from a point.
11.2 Visibility: the 'visibility' property
'visibility'
Value: visible | hidden | collapse | inherit
Initial: visible
Applies to: all elements
Inherited: yes
Percentages: N/A
Media: visual
Computed value: as specified
The 'visibility' property specifies whether the boxes generated by an
element are rendered. Invisible boxes still affect layout (set the
'display' property to 'none' to suppress box generation altogether).
Values have the following meanings:
visible
The generated box is visible.
hidden
The generated box is invisible (fully transparent, nothing is
drawn), but still affects layout. Furthermore, descendents of
the element will be visible if they have 'visibility: visible'.
collapse
Please consult the section on dynamic row and column effects in
tables. If used on elements other than rows, row groups,
columns, or column groups, 'collapse' has the same meaning as
'hidden'.
This property may be used in conjunction with scripts to create
dynamic effects.
In the following example, pressing either form button invokes an
author-defined script function that causes the corresponding box to
become visible and the other to be hidden. Since these boxes have the
same size and position, the effect is that one replaces the other.
(The script code is in a hypothetical script language. It may or may
not have any effect in a CSS-capable UA.)
Dynamic visibility example

Choose a suspect:

Name: Al Capone

Residence: Chicago

Name: Lucky Luciano

Residence: New York

_________________________________________________________________
_________________________________________________________________
12 Generated content, automatic numbering, and lists
Contents
* 12.1 The :before and :after pseudo-elements
* 12.2 The 'content' property
* 12.3 Quotation marks
+ 12.3.1 Specifying quotes with the 'quotes' property
+ 12.3.2 Inserting quotes with the 'content' property
* 12.4 Automatic counters and numbering
+ 12.4.1 Nested counters and scope
+ 12.4.2 Counter styles
+ 12.4.3 Counters in elements with 'display: none'
* 12.5 Lists
+ 12.5.1 Lists: the 'list-style-type', 'list-style-image',
'list-style-position', and 'list-style' properties
In some cases, authors may want user agents to render content that
does not come from the document tree. One familiar example of this is
a numbered list; the author does not want to list the numbers
explicitly, he or she wants the user agent to generate them
automatically. Similarly, authors may want the user agent to insert
the word "Figure" before the caption of a figure, or "Chapter 7"
before the seventh chapter title. For audio or braille in particular,
user agents should be able to insert these strings.
In CSS 2.1, content may be generated by two mechanisms:
* The 'content' property, in conjunction with the :before and :after
pseudo-elements.
* Elements with a value of 'list-item' for the 'display' property.
12.1 The :before and :after pseudo-elements
Authors specify the style and location of generated content with the
:before and :after pseudo-elements. As their names indicate, the
:before and :after pseudo-elements specify the location of content
before and after an element's document tree content. The 'content'
property, in conjunction with these pseudo-elements, specifies what is
inserted.
Example(s):
For example, the following rule inserts the string "Note: " before the
content of every P element whose "class" attribute has the value
"note":
p.note:before { content: "Note: " }
The formatting objects (e.g., boxes) generated by an element include
generated content. So, for example, changing the above style sheet to:
p.note:before { content: "Note: " }
p.note { border: solid green }
would cause a solid green border to be rendered around the entire
paragraph, including the initial string.
The :before and :after pseudo-elements inherit any inheritable
properties from the element in the document tree to which they are
attached.
Example(s):
For example, the following rules insert an open quote mark before
every Q element. The color of the quote mark will be red, but the font
will be the same as the font of the rest of the Q element:
q:before {
content: open-quote;
color: red
}
In a :before or :after pseudo-element declaration, non-inherited
properties take their initial values.
Example(s):
So, for example, because the initial value of the 'display' property
is 'inline', the quote in the previous example is inserted as an
inline box (i.e., on the same line as the element's initial text
content). The next example explicitly sets the 'display' property to
'block', so that the inserted text becomes a block:
body:after {
content: "The End";
display: block;
margin-top: 2em;
text-align: center;
}
The :before and :after pseudo-elements elements interact with other
boxes, such as run-in boxes, as if they were real elements inserted
just inside their associated element.
Example(s):
For example, the following document fragment and style sheet:

Header

h2 { display: run-in; }

Text

p:before { display: block; content: 'Some'; }
...would render in exactly the same way as the following document
fragment and style sheet:

Header

Header

...would render in exactly the same way as the following document
fragment and style sheet:

Header Thing

h2 { display: block; }
span { display: block; }

Text

Note. This specification does not fully define the interaction of
:before and :after with replaced elements (such as IMG in HTML). This
will be defined in more detail in a future specification.
12.2 The 'content' property
'content'
Value: normal | none | [ | | |
attr() | open-quote | close-quote | no-open-quote |
no-close-quote ]+ | inherit
Initial: normal
Applies to: :before and :after pseudo-elements
Inherited: no
Percentages: N/A
Media: all
Computed value: On elements, always computes to 'normal'. On :before
and :after, if 'normal' is specified, computes to 'none'. Otherwise,
for URI values, the absolute URI; for attr() values, the resulting
string; for other keywords, as specified.
This property is used with the :before and :after pseudo-elements to
generate content in a document. Values have the following meanings:
none
The pseudo-element is not generated.
normal
Computes to 'none' for the :before and :after pseudo-elements.
Text content (see the section on strings).
The value is a URI that designates an external resource (such
as an image). If the user agent cannot display the resource it
must either leave it out as if it were not specified or display
some indication that the resource cannot be displayed.
Counters may be specified with two different functions:
'counter()' or 'counters()'. The former has two forms:
'counter(name)' or 'counter(name, style)'. The generated text
is the value of the innermost counter of the given name in
scope at this pseudo-element; it is formatted in the indicated
style ('decimal' by default). The latter function also has two
forms: 'counters(name, string)' or 'counters(name, string,
style)'. The generated text is the value of all counters with
the given name in scope at this pseudo-element, from outermost
to innermost separated by the specified string. The counters
are rendered in the indicated style ('decimal' by default). See
the section on automatic counters and numbering for more
information.
open-quote and close-quote
These values are replaced by the appropriate string from the
'quotes' property.
no-open-quote and no-close-quote
Introduces no content, but increments (decrements) the level of
nesting for quotes.
attr(X)
This function returns as a string the value of attribute X for
the subject of the selector. The string is not parsed by the
CSS processor. If the subject of the selector doesn't have an
attribute X, an empty string is returned. The case-sensitivity
of attribute names depends on the document language.
Note. In CSS 2.1, it is not possible to refer to attribute values for
other elements than the subject of the selector.
The 'display' property controls whether the content is placed in a
block or inline box.
Example(s):
The following rule causes the string "Chapter: " to be generated
before each H1 element:
H1:before {
content: "Chapter: ";
display: inline;
}
Authors may include newlines in the generated content by writing the
"\A" escape sequence in one of the strings after the 'content'
property. This inserted line break is still subject to the
'white-space' property. See "Strings" and "Characters and case" for
more information on the "\A" escape sequence.
Example(s):
h1:before {
display: block;
text-align: center;
white-space: pre;
content: "chapter\A hoofdstuk\A chapitre"
}
Generated content does not alter the document tree. In particular, it
is not fed back to the document language processor (e.g., for
reparsing).
12.3 Quotation marks
In CSS 2.1, authors may specify, in a style-sensitive and
context-dependent manner, how user agents should render quotation
marks. The 'quotes' property specifies pairs of quotation marks for
each level of embedded quotation. The 'content' property gives access
to those quotation marks and causes them to be inserted before and
after a quotation.
12.3.1 Specifying quotes with the 'quotes' property
'quotes'
Value: []+ | none | inherit
Initial: depends on user agent
Applies to: all elements
Inherited: yes
Percentages: N/A
Media: visual
Computed value: as specified
This property specifies quotation marks for any number of embedded
quotations. Values have the following meanings:
none
The 'open-quote' and 'close-quote' values of the 'content'
property produce no quotation marks.
[]+
Values for the 'open-quote' and 'close-quote' values of the
'content' property are taken from this list of pairs of
quotation marks (opening and closing). The first (leftmost)
pair represents the outermost level of quotation, the second
pair the first level of embedding, etc. The user agent must
apply the appropriate pair of quotation marks according to the
level of embedding.
Example(s):
For example, applying the following style sheet:
/* Specify pairs of quotes for two levels in two languages */
q:lang(en) { quotes: '"' '"' "'" "'" }
q:lang(no) { quotes: "«" "»" '"' '"' }
/* Insert quotes before and after Q element content */
q:before { content: open-quote }
q:after { content: close-quote }
to the following HTML fragment:
Quotes

Quote me!
would allow a user agent to produce:
"Quote me!"
while this HTML fragment:

Quotes

Trøndere gråter når Vinsjan på kaia blir deklamert.
would produce:
«Trøndere gråter når "Vinsjan på kaia" blir deklamert.»
Note. While the quotation marks specified by 'quotes' in the previous
examples are conveniently located on computer keyboards, high quality
typesetting would require different ISO 10646 characters. The
following informative table lists some of the ISO 10646 quotation mark
characters:
Character Approximate rendering ISO 10646 code (hex) Description
" " 0022 QUOTATION MARK [the ASCII double quotation mark]
' ' 0027 APOSTROPHE [the ASCII single quotation mark]
< < 2039 SINGLE LEFT-POINTING ANGLE QUOTATION MARK
> > 203A SINGLE RIGHT-POINTING ANGLE QUOTATION MARK
« « 00AB LEFT-POINTING DOUBLE ANGLE QUOTATION MARK
» » 00BB RIGHT-POINTING DOUBLE ANGLE QUOTATION MARK
` ` 2018 LEFT SINGLE QUOTATION MARK [single high-6]
' ' 2019 RIGHT SINGLE QUOTATION MARK [single high-9]
" `` 201C LEFT DOUBLE QUOTATION MARK [double high-6]
" '' 201D RIGHT DOUBLE QUOTATION MARK [double high-9]
" ,, 201E DOUBLE LOW-9 QUOTATION MARK [double low-9]
12.3.2 Inserting quotes with the 'content' property
Quotation marks are inserted in appropriate places in a document with
the 'open-quote' and 'close-quote' values of the 'content' property.
Each occurrence of 'open-quote' or 'close-quote' is replaced by one of
the strings from the value of 'quotes', based on the depth of nesting.
'Open-quote' refers to the first of a pair of quotes, 'close-quote'
refers to the second. Which pair of quotes is used depends on the
nesting level of quotes: the number of occurrences of 'open-quote' in
all generated text before the current occurrence, minus the number of
occurrences of 'close-quote'. If the depth is 0, the first pair is
used, if the depth is 1, the second pair is used, etc. If the depth is
greater than the number of pairs, the last pair is repeated. A
'close-quote' or 'no-close-quote' that would make the depth negative
is in error and is ignored (at rendering time): the depth stays at 0
and no quote mark is rendered (although the rest of the 'content'
property's value is still inserted).
Note. The quoting depth is independent of the nesting of the source
document or the formatting structure.
Some typographic styles require open quotation marks to be repeated
before every paragraph of a quote spanning several paragraphs, but
only the last paragraph ends with a closing quotation mark. In CSS,
this can be achieved by inserting "phantom" closing quotes. The
keyword 'no-close-quote' decrements the quoting level, but does not
insert a quotation mark.
Example(s):
The following style sheet puts opening quotation marks on every
paragraph in a BLOCKQUOTE, and inserts a single closing quote at the
end:
blockquote p:before { content: open-quote }
blockquote p:after { content: no-close-quote }
blockquote p.last:after { content: close-quote }
This relies on the last paragraph being marked with a class "last".
For symmetry, there is also a 'no-open-quote' keyword, which inserts
nothing, but increments the quotation depth by one.
12.4 Automatic counters and numbering
Automatic numbering in CSS 2.1 is controlled with two properties,
'counter-increment' and 'counter-reset'. The counters defined by these
properties are used with the counter() and counters() functions of the
the 'content' property.
'counter-reset'
Value: [ ? ]+ | none | inherit
Initial: none
Applies to: all elements
Inherited: no
Percentages: N/A
Media: all
Computed value: as specified
'counter-increment'
Value: [ ? ]+ | none | inherit
Initial: none
Applies to: all elements
Inherited: no
Percentages: N/A
Media: all
Computed value: as specified
The 'counter-increment' property accepts one or more names of counters
(identifiers), each one optionally followed by an integer. The integer
indicates by how much the counter is incremented for every occurrence
of the element. The default increment is 1. Zero and negative integers
are allowed.
The 'counter-reset' property also contains a list of one or more names
of counters, each one optionally followed by an integer. The integer
gives the value that the counter is set to on each occurrence of the
element. The default is 0.
Example(s):
This example shows a way to number chapters and sections with "Chapter
1", "1.1", "1.2", etc.
BODY {
counter-reset: chapter; /* Create a chapter counter scope */
}
H1:before {
content: "Chapter " counter(chapter) ". ";
counter-increment: chapter; /* Add 1 to chapter */
}
H1 {
counter-reset: section; /* Set section to 0 */
}
H2:before {
content: counter(chapter) "." counter(section) " ";
counter-increment: section;
}
If an element increments/resets a counter and also uses it (in the
'content' property of its :before or :after pseudo-element), the
counter is used after being incremented/reset.
If an element both resets and increments a counter, the counter is
reset first and then incremented.
If the same counter is specified more than once in the value of the
'counter-reset' and 'counter-increment' properties, each
reset/increment of the counter is processed in the order specified.
Example(s):
The following example will reset the 'section' counter to 0:
H1 { counter-reset: section 2 section }
The following example will increment the 'chapter' counter by 3:
H1 { counter-increment: chapter chapter 2 }
The 'counter-reset' property follows the cascading rules. Thus, due to
cascading, the following style sheet:
H1 { counter-reset: section -1 }
H1 { counter-reset: imagenum 99 }
will only reset 'imagenum'. To reset both counters, they have to be
specified together:
H1 { counter-reset: section -1 imagenum 99 }
12.4.1 Nested counters and scope
Counters are "self-nesting", in the sense that resetting a counter in
a descendant element or pseudo-element automatically creates a new
instance of the counter. This is important for situations like lists
in HTML, where elements can be nested inside themselves to arbitrary
depth. It would be impossible to define uniquely named counters for
each level.
Example(s):
Thus, the following suffices to number nested list items. The result
is very similar to that of setting 'display:list-item' and
'list-style: inside' on the LI element:
OL { counter-reset: item }
LI { display: block }
LI:before { content: counter(item) ". "; counter-increment: item }
The scope of a counter starts at the first element in the document
that has a 'counter-reset' for that counter and includes the element's
descendants and its following siblings with their descendants.
However, it does not include any elements in the scope of a counter
with the same name created by a 'counter-reset' on a later sibling of
the element or by a later 'counter-reset' on the same element.
If 'counter-increment' or 'content' on an element or pseudo-element
refers to a counter that is not in the scope of any 'counter-reset',
implementations should behave as though a 'counter-reset' had reset
the counter to 0 on that element or pseudo-element.
In the example above, an OL will create a counter, and all children of
the OL will refer to that counter.
If we denote by item[n] the n^th instance of the "item" counter, and
by "{" and "}" the beginning and end of a scope, then the following
HTML fragment will use the indicated counters. (We assume the style
sheet as given in the example above).

item

item

item

item

item

item

item

item

item

item

item

item

Example(s):
Another example, showing how scope works when counters are used on
elements that are not nested, is the following. This shows how the
style rules given above to number chapters and sections would apply to
the markup given.

might produce something like this:
i This is the first item.
ii This is the second item.
iii This is the third item.
The list marker alignment (here, right justified) depends on the user
agent.
'list-style-image'
Value: | none | inherit
Initial: none
Applies to: elements with 'display: list-item'
Inherited: yes
Percentages: N/A
Media: visual
Computed value: absolute URI or 'none'
This property sets the image that will be used as the list item
marker. When the image is available, it will replace the marker set
with the 'list-style-type' marker.
Example(s):
The following example sets the marker at the beginning of each list
item to be the image "ellipse.png".
ul { list-style-image: url("http://png.com/ellipse.png") }
'list-style-position'
Value: inside | outside | inherit
Initial: outside
Applies to: elements with 'display: list-item'
Inherited: yes
Percentages: N/A
Media: visual
Computed value: as specified
This property specifies the position of the marker box in the
principal block box. Values have the following meanings:
outside
The marker box is outside the principal block box. CSS 2.1 does
not specify the precise location of the marker box.
inside
The marker box is the first inline box in the principal block
box, after which the element's content flows. CSS 2.1 does not
specify the precise location of the marker box.
For example:
Comparison of inside/outside position

first list item comes first

second list item comes second

first list item comes first

second list item comes second

The above example may be formatted as:
Difference between inside and outside list style position [D]
In right-to-left text, the markers would have been on the right side
of the box.
'list-style'
Value: [ || ||
] | inherit
Initial: see individual properties
Applies to: elements with 'display: list-item'
Inherited: yes
Percentages: N/A
Media: visual
Computed value: see individual properties
The 'list-style' property is a shorthand notation for setting the
three properties 'list-style-type', 'list-style-image', and
'list-style-position' at the same place in the style sheet.
Example(s):
ul { list-style: upper-roman inside } /* Any "ul" element */
ul > li > ul { list-style: circle outside } /* Any "ul" child
of an "li" child
of a "ul" element */
Although authors may specify 'list-style' information directly on list
item elements (e.g., "li" in HTML), they should do so with care. The
following rules look similar, but the first declares a descendant
selector and the second a (more specific) child selector.
ol.alpha li { list-style: lower-alpha } /* Any "li" descendant of an "ol" */
ol.alpha > li { list-style: lower-alpha } /* Any "li" child of an "ol" */
Authors who use only the descendant selector may not achieve the
results they expect. Consider the following rules:
WARNING: Unexpected results due to cascade

level 1

level 2

The desired rendering would have level 1 list items with 'lower-alpha'
labels and level 2 items with 'disc' labels. However, the cascading
order will cause the first style rule (which includes specific class
information) to mask the second. The following rules solve the problem
by employing a child selector instead:
ol.alpha > li { list-style: lower-alpha }
ul li { list-style: disc }
Another solution would be to specify 'list-style' information only on
the list type elements:
ol.alpha { list-style: lower-alpha }
ul { list-style: disc }
Inheritance will transfer the 'list-style' values from OL and UL
elements to LI elements. This is the recommended way to specify list
style information.
Example(s):
A URI value may be combined with any other value, as in:
ul { list-style: url("http://png.com/ellipse.png") disc }
In the example above, the 'disc' will be used when the image is
unavailable.
A value of 'none' for the 'list-style' property sets both
'list-style-type' and 'list-style-image' to 'none':
ul { list-style: none }
The result is that no list-item marker is displayed.
_________________________________________________________________
_________________________________________________________________
13 Paged media
Contents
* 13.1 Introduction to paged media
* 13.2 Page boxes: the @page rule
+ 13.2.1 Page margins
o 13.2.1.1 Rendering page boxes that do not fit a target
sheet
o 13.2.1.2 Positioning the page box on the sheet
+ 13.2.2 Page selectors: selecting left, right, and first pages
+ 13.2.3 Content outside the page box
* 13.3 Page breaks
+ 13.3.1 Page break properties: 'page-break-before',
'page-break-after', 'page-break-inside'
+ 13.3.2 Breaks inside elements: 'orphans', 'widows'
+ 13.3.3 Allowed page breaks
+ 13.3.4 Forced page breaks
+ 13.3.5 "Best" page breaks
* 13.4 Cascading in the page context
13.1 Introduction to paged media
Paged media (e.g., paper, transparencies, pages that are displayed on
computer screens, etc.) differ from continuous media in that the
content of the document is split into one or more discrete pages. To
handle pages, CSS 2.1 describes how page margins are set on page
boxes, and how page breaks are declared.
The user agent is responsible for transferring the page boxes of a
document onto the real sheets where the document will ultimately be
rendered (paper, transparency, screen, etc.). There is often a 1-to-1
relationship between a page box and a sheet, but this is not always
the case. Transfer possibilities include:
* Transferring one page box to one sheet (e.g., single-sided
printing).
* Transferring two page boxes to both sides of the same sheet (e.g.,
double-sided printing).
* Transferring N (small) page boxes to one sheet (called "n-up").
* Transferring one (large) page box to N x M sheets (called
"tiling").
* Creating signatures. A signature is a group of pages printed on a
sheet, which, when folded and trimmed like a book, appear in their
proper sequence.
* Printing one document to several output trays.
* Outputting to a file.
13.2 Page boxes: the @page rule
The page box is a rectangular region that contains two areas:
* The page area. The page area includes the boxes laid out on that
page. The edges of the first page area establish the rectangle
that is the initial containing block of the document.
* The margin area, which surrounds the page area.
The size of a page box cannot be specified in CSS 2.1.
Authors can specify the margins of a page box inside an @page rule. An
@page rule consists of the keyword "@page", followed by an optional
page selector, followed by a block of declarations. The declarations
in an @page rule are said to be in the page context.
The page selector specifies for which pages the declarations apply. In
CSS 2.1, page selectors may designate the first page, all left pages,
or all right pages
13.2.1 Page margins
In CSS 2.1, only the margin properties ('margin-top', 'margin-right',
'margin-bottom', 'margin-left', and 'margin') apply within the page
context. The following diagram shows the relationships between the
sheet, page box, and page margins:
Illustration of sheet, page box, margin, and page area. [D]
Example(s):
Here is a simple example which sets all page margins on all pages:
@page {
margin: 3cm;
}
The page context has no notion of fonts, so 'em' and 'ex' units are
not allowed. Percentage values on the margin properties are relative
to the dimensions of the page box; for left and right margins, they
refer to the width of the page box while for top and bottom margins,
they refer to the height of the page box. All other units associated
with the respective CSS 2.1 properties are allowed.
Due to negative margin values (either on the page box or on elements)
or absolute positioning content may end up outside the page box, but
this content may be "cut" -- by the user agent, the printer, or
ultimately, the paper cutter.
13.2.1.1 Rendering page boxes that do not fit a target sheet
If a page box does not fit the target sheet dimensions, the user agent
may choose to:
* Rotate the page box 90ï¿½ if this will make the page box fit.
* Scale the page to fit the target.
The user agent should consult the user before performing these
operations.
13.2.1.2 Positioning the page box on the sheet
When the page box is smaller than the target size, the user agent is
free to place the page box anywhere on the sheet. However, it is
recommended that the page box be centered on the sheet since this will
align double-sided pages and avoid accidental loss of information that
is printed near the edge of the sheet.
13.2.2 Page selectors: selecting left, right, and first pages
When printing double-sided documents, the page boxes on left and right
pages may be different. This can be expressed through two CSS
pseudo-classes that may be used in page selectors.
All pages are automatically classified by user agents into either the
:left or :right pseudo-class.
Example(s):
@page :left {
margin-left: 4cm;
margin-right: 3cm;
}
@page :right {
margin-left: 3cm;
margin-right: 4cm;
}
If different declarations have been given for left and right pages,
the user agent must honor these declarations even if the user agent
does not transfer the page boxes to left and right sheets (e.g., a
printer that only prints single-sided).
Authors may also specify style for the first page of a document with
the :first pseudo-class:
Example(s):
@page { margin: 2cm } /* All margins set to 2cm */
@page :first {
margin-top: 10cm /* Top margin on first page 10cm */
}
Properties specified in a :left or :right @page rule override those
specified in an @page rule that has no pseudo-class specified.
Properties specified in a :first @page rule override those specified
in :left or :right @page rules.
Margin declarations on left, right, and first pages may result in
different page area widths. To simplify implementations, user agents
may use a single page area width on left, right, and first pages. In
this case, the page area width of the first page should be used.
13.2.3 Content outside the page box
When formatting content in the page model, some content may end up
outside the page box. For example, an element whose 'white-space'
property has the value 'pre' may generate a box that is wider than the
page box. Also, when boxes are positioned absolutely, they may end up
in "inconvenient" locations. For example, images may be placed on the
edge of the page box or 100,000 meters below the page box.
The exact formatting of such elements lies outside the scope of this
specification. However, we recommend that authors and user agents
observe the following general principles concerning content outside
the page box:
* Content should be allowed slightly beyond the page box to allow
pages to "bleed".
* User agents should avoid generating a large number of empty page
boxes to honor the positioning of elements (e.g., you don't want
to print 100 blank pages).
* Authors should not position elements in inconvenient locations
just to avoid rendering them.
* User agents may handle boxes positioned outside the page box in
several ways, including discarding them or creating page boxes for
them at the end of the document.
13.3 Page breaks
This section describes page breaks in CSS 2.1. Five properties
indicate where the user agent may or should break pages, and on what
page (left or right) the subsequent content should resume. Each page
break ends layout in the current page box and causes remaining pieces
of the document tree to be laid out in a new page box.
13.3.1 Page break properties: 'page-break-before', 'page-break-after',
'page-break-inside'
'page-break-before'
Value: auto | always | avoid | left | right | inherit
Initial: auto
Applies to: block-level elements
Inherited: no
Percentages: N/A
Media: visual, paged
Computed value: as specified
'page-break-after'
Value: auto | always | avoid | left | right | inherit
Initial: auto
Applies to: block-level elements
Inherited: no
Percentages: N/A
Media: visual, paged
Computed value: as specified
'page-break-inside'
Value: avoid | auto | inherit
Initial: auto
Applies to: block-level elements
Inherited: yes
Percentages: N/A
Media: visual, paged
Computed value: as specified
Values for these properties have the following meanings:
auto
Neither force nor forbid a page break before (after, inside)
the generated box.
always
Always force a page break before (after) the generated box.
avoid
Avoid a page break before (after, inside) the generated box.
left
Force one or two page breaks before (after) the generated box
so that the next page is formatted as a left page.
right
Force one or two page breaks before (after) the generated box
so that the next page is formatted as a right page.
Whether the first page of a document is :left or :right depends on the
major writing direction of the document. A conforming user agent may
interpret the values 'left' and 'right' as 'always'.
A potential page break location is typically under the influence of
the parent element's 'page-break-inside' property, the
'page-break-after' property of the preceding element, and the
'page-break-before' property of the following element. When these
properties have values other than 'auto', the values 'always', 'left',
and 'right' take precedence over 'avoid'.
User Agents must apply these properties to block-level elements in the
normal flow of the root element. User agents may also apply these
properties to other elements, e.g., 'table-row' elements.
When a page break splits a box, the box's margins, borders, and
padding have no visual effect where the split occurs.
13.3.2 Breaks inside elements: 'orphans', 'widows'
'orphans'
Value: | inherit
Initial: 2
Applies to: block-level elements
Inherited: yes
Percentages: N/A
Media: visual, paged
Computed value: as specified
'widows'
Value: | inherit
Initial: 2
Applies to: block-level elements
Inherited: yes
Percentages: N/A
Media: visual, paged
Computed value: as specified
The 'orphans' property specifies the minimum number of lines of a
paragraph that must be left at the bottom of a page. The 'widows'
property specifies the minimum number of lines of a paragraph that
must be left at the top of a page. Examples of how they are used to
control page breaks are given below.
For information about paragraph formatting, please consult the section
on line boxes.
13.3.3 Allowed page breaks
In the normal flow, page breaks can occur at the following places:
1. In the vertical margin between block boxes. When a page break
occurs here, the used values of the relevant 'margin-top' and
'margin-bottom' properties are set to '0'.
2. Between line boxes inside a block box.
These breaks are subject to the following rules:
* Rule A: Breaking at (1) is allowed only if the 'page-break-after'
and 'page-break-before' properties of all the elements generating
boxes that meet at this margin allow it, which is when at least
one of them has the value 'always', 'left', or 'right', or when
all of them are 'auto'.
* Rule B: However, if all of them are 'auto' and the nearest common
ancestor of all the elements has a 'page-break-inside' value of
'avoid', then breaking here is not allowed.
* Rule C: Breaking at (2) is allowed only if the number of line
boxes between the break and the start of the enclosing block box
is the value of 'orphans' or more, and the number of line boxes
between the break and the end of the box is the value of 'widows'
or more.
* Rule D: In addition, breaking at (2) is allowed only if the
'page-break-inside' property is 'auto'.
If the above doesn't provide enough break points to keep content from
overflowing the page boxes, then rules A, B and D are dropped in order
to find additional breakpoints.
If that still does not lead to sufficient break points, rule C is
dropped as well, to find still more break points.
13.3.4 Forced page breaks
A page break must occur at (1) if, among the 'page-break-after' and
'page-break-before' properties of all the elements generating boxes
that meet at this margin, there is at least one with the value
'always', 'left', or 'right'.
13.3.5 "Best" page breaks
CSS 2.1 does not define which of a set of allowed page breaks must be
used; CSS 2.1 does not forbid a user agent from breaking at every
possible break point, or not to break at all. But CSS 2.1 does
recommend that user agents observe the following heuristics (while
recognizing that they are sometimes contradictory):
* Break as few times as possible.
* Make all pages that don't end with a forced break appear to have
about the same height.
* Avoid breaking inside a block that has a border.
* Avoid breaking inside a table.
* Avoid breaking inside a floated element
Example(s):
Suppose, for example, that the style sheet contains 'orphans: 4',
'widows: 2', and there are 20 lines (line boxes) available at the
bottom of the current page:
* If a paragraph at the end of the current page contains 20 lines or
fewer, it should be placed on the current page.
* If the paragraph contains 21 or 22 lines, the second part of the
paragraph must not violate the 'widows' constraint, and so the
second part must contain exactly two lines
* If the paragraph contains 23 lines or more, the first part should
contain 20 lines and the second part the remaining lines.
Now suppose that 'orphans' is '10', 'widows' is '20', and there are 8
lines available at the bottom of the current page:
* If a paragraph at the end of the current page contains 8 lines or
fewer, it should be placed on the current page.
* If the paragraph contains 9 lines or more, it cannot be split
(that would violate the orphan constraint), so it should move as a
block to the next page.
13.4 Cascading in the page context
Declarations in the page context obey the cascade just like normal CSS
declarations.
Example(s):
Consider the following example:
@page {
margin-left: 3cm;
}
@page :left {
margin-left: 4cm;
}
Due to the higher specificity of the pseudo-class selector, the left
margin on left pages will be '4cm' and all other pages (i.e., the
right pages) will have a left margin of '3cm'.
_________________________________________________________________
_________________________________________________________________
14 Colors and Backgrounds
Contents
* 14.1 Foreground color: the 'color' property
* 14.2 The background
+ 14.2.1 Background properties: 'background-color',
'background-image', 'background-repeat',
'background-attachment', 'background-position', and
'background'
* 14.3 Gamma correction
CSS properties allow authors to specify the foreground color and
background of an element. Backgrounds may be colors or images.
Background properties allow authors to position a background image,
repeat it, and declare whether it should be fixed with respect to the
viewport or scrolled along with the document.
See the section on color units for the syntax of valid color values.
14.1 Foreground color: the 'color' property
'color'
Value: | inherit
Initial: depends on user agent
Applies to: all elements
Inherited: yes
Percentages: N/A
Media: visual
Computed value: as specified
This property describes the foreground color of an element's text
content. There are different ways to specify red:
Example(s):
em { color: red } /* predefined color name */
em { color: rgb(255,0,0) } /* RGB range 0-255 */
14.2 The background
Authors may specify the background of an element (i.e., its rendering
surface) as either a color or an image. In terms of the box model,
"background" refers to the background of the content, padding and
border areas. Border colors and styles are set with the border
properties. Margins are always transparent.
Background properties are not inherited, but the parent box's
background will shine through by default because of the initial
'transparent' value on 'background-color'.
The background of the root element becomes the background of the
canvas and covers the entire canvas, anchored (for
'background-position') at the same point as it would be if it was
painted only for the root element itself. The root element does not
paint this background again.
For HTML documents, however, we recommend that authors specify the
background for the BODY element rather than the HTML element. For HTML
documents whose root HTML element has computed values of 'transparent'
for 'background-color' and 'none' for 'background-image', user agents
must instead use the computed value of those properties from that HTML
element's first BODY element child when painting backgrounds for the
canvas, and must not paint a background for that BODY element. Such
backgrounds must also be anchored at the same point as they would be
if they were painted only for the root element. This does not apply to
XHTML documents.
According to these rules, the canvas underlying the following HTML
document will have a "marble" background:
Setting the canvas background

My background is marble.
Note that the rule for the BODY element will work even though the BODY
tag has been omitted in the HTML source since the HTML parser will
infer the missing tag.
Backgrounds of elements that form a stacking context (see the
'z-index' property) are painted at the bottom of the element's
stacking context, below anything in that stacking context.
14.2.1 Background properties: 'background-color', 'background-image',
'background-repeat', 'background-attachment', 'background-position', and
'background'
'background-color'
Value: | transparent | inherit
Initial: transparent
Applies to: all elements
Inherited: no
Percentages: N/A
Media: visual
Computed value: as specified
This property sets the background color of an element, either a
value or the keyword 'transparent', to make the underlying
colors shine through.
Example(s):
h1 { background-color: #F00 }
'background-image'
Value: | none | inherit
Initial: none
Applies to: all elements
Inherited: no
Percentages: N/A
Media: visual
Computed value: absolute URI or none
This property sets the background image of an element. When setting a
background image, authors should also specify a background color that
will be used when the image is unavailable. When the image is
available, it is rendered on top of the background color. (Thus, the
color is visible in the transparent parts of the image).
Values for this property are either , to specify the image, or
'none', when no image is used.
Example(s):
body { background-image: url("marble.png") }
p { background-image: none }
Intrinsic dimensions expressed as percentages must be resolved
relative to the dimensions of the rectangle that establishes the
coordinate system for the 'background-position' property.
If the image has no intrinsic dimensions and has an intrinsic ratio
the dimensions must be assumed to be the largest dimensions at that
ratio such that neither dimension exceeds the dimensions of the
rectangle that establishes the coordinate system for the
'background-position' property.
If the image has no intrinsic ratio either, then the dimensions must
be assumed to be the rectangle that establishes the coordinate system
for the 'background-position' property.
'background-repeat'
Value: repeat | repeat-x | repeat-y | no-repeat | inherit
Initial: repeat
Applies to: all elements
Inherited: no
Percentages: N/A
Media: visual
Computed value: as specified
If a background image is specified, this property specifies whether
the image is repeated (tiled), and how. All tiling covers the content,
padding and border areas of a box.
The tiling and positioning of the background-image on inline elements
is undefined in this specification. A future level of CSS may define
the tiling and positioning of the background-image on inline elements.
Values have the following meanings:
repeat
The image is repeated both horizontally and vertically.
repeat-x
The image is repeated horizontally only.
repeat-y
The image is repeated vertically only.
no-repeat
The image is not repeated: only one copy of the image is drawn.
Example(s):
body {
background: white url("pendant.png");
background-repeat: repeat-y;
background-position: center;
}
A centered background image, with copies repeated up and down the
padding and content areas. [D]
One copy of the background image is centered, and other copies are put
above and below it to make a vertical band behind the element.
'background-attachment'
Value: scroll | fixed | inherit
Initial: scroll
Applies to: all elements
Inherited: no
Percentages: N/A
Media: visual
Computed value: as specified
If a background image is specified, this property specifies whether it
is fixed with regard to the viewport ('fixed') or scrolls along with
the containing block ('scroll').
Note that there is only one viewport per view. If an element has a
scrolling mechanism (see 'overflow'), a 'fixed' background doesn't
move with the element, and a 'scroll' background doesn't move with the
scrolling mechanism.
Even if the image is fixed, it is still only visible when it is in the
content, padding or border area of the element. Thus, unless the image
is tiled ('background-repeat: repeat'), it may be invisible.
Example(s):
This example creates an infinite vertical band that remains "glued" to
the viewport when the element is scrolled.
body {
background: red url("pendant.png");
background-repeat: repeat-y;
background-attachment: fixed;
}
User agents that do not support 'fixed' backgrounds (for example due
to limitations of the hardware platform) should ignore declarations
with the keyword 'fixed'. For example:
body {
background: white url(paper.png) scroll; /* for all UAs */
background: white url(ledger.png) fixed; /* for UAs that do fixed backgrounds
*/
}
See the section on conformance for details.
'background-position'
Value: [ [ | | left | center | right ] [
| | top | center | bottom ]? ] | [ [ left |
center | right ] || [ top | center | bottom ] ] | inherit
Initial: 0% 0%
Applies to: all elements
Inherited: no
Percentages: refer to the size of the box itself
Media: visual
Computed value: for the absolute value, otherwise a
percentage
If a background image has been specified, this property specifies its
initial position. If only one value is specified, the second value is
assumed to be 'center'. If at least one value is not a keyword, then
the first value represents the horizontal position and the second
represents the vertical position. Negative and
values are allowed.
A percentage X aligns the point X% across (for horizontal) or
down (for vertical) the image with the point X% across (for
horizontal) or down (for vertical) the element's padding box.
For example, with a value pair of '0% 0%',the upper left corner
of the image is aligned with the upper left corner of the
padding box. A value pair of '100% 100%' places the lower right
corner of the image in the lower right corner of the padding
box. With a value pair of '14% 84%', the point 14% across and
84% down the image is to be placed at the point 14% across and
84% down the padding box.
A length L aligns the top left corner of the image a distance L
to the right of (for horizontal) or below (for vertical) the
top left corner of the element's padding box. For example, with
a value pair of '2cm 1cm', the upper left corner of the image
is placed 2cm to the right and 1cm below the upper left corner
of the padding box.
top
Equivalent to '0%' for the vertical position.
right
Equivalent to '100%' for the horizontal position.
bottom
Equivalent to '100%' for the vertical position.
left
Equivalent to '0%' for the horizontal position.
center
Equivalent to '50%' for the horizontal position if it is not
otherwise given, or '50%' for the vertical position if it is.
Example(s):
body { background: url("banner.jpeg") right top } /* 100% 0% */
body { background: url("banner.jpeg") top center } /* 50% 0% */
body { background: url("banner.jpeg") center } /* 50% 50% */
body { background: url("banner.jpeg") bottom } /* 50% 100% */
The tiling and positioning of the background-image on inline elements
is undefined in this specification. A future level of CSS may define
the tiling and positioning of the background-image on inline elements.
If the background image is fixed within the viewport (see the
'background-attachment' property), the image is placed relative to the
viewport instead of the element's padding box. For example,
Example(s):
body {
background-image: url("logo.png");
background-attachment: fixed;
background-position: 100% 100%;
background-repeat: no-repeat;
}
In the example above, the (single) image is placed in the lower-right
corner of the viewport.
'background'
Value: [ || ||
|| ||
] | inherit
Initial: see individual properties
Applies to: all elements
Inherited: no
Percentages: allowed on 'background-position'
Media: visual
Computed value: see individual properties
The 'background' property is a shorthand property for setting the
individual background properties (i.e., 'background-color',
'background-image', 'background-repeat', 'background-attachment' and
'background-position') at the same place in the style sheet.
Given a valid declaration, the 'background' property first sets all
the individual background properties to their initial values, then
assigns explicit values given in the declaration.
Example(s):
In the first rule of the following example, only a value for
'background-color' has been given and the other individual properties
are set to their initial value. In the second rule, all individual
properties have been specified.
BODY { background: red }
P { background: url("chess.png") gray 50% repeat fixed }
14.3 Gamma correction
For information about gamma issues, please consult the Gamma Tutorial
in the PNG specification ([PNG]).
Note. In the computation of gamma correction, UAs displaying on a CRT
may assume an ideal CRT and ignore any effects on apparent gamma
caused by dithering. That means the minimal handling they need to do
on current platforms is:
PC using MS-Windows
none
Unix using X11
none
Mac using QuickDraw
apply gamma 1.45 [ICC42] (ColorSync-savvy applications may
simply pass the sRGB ICC profile to ColorSync to perform
correct color correction)
SGI using X
apply the gamma value from /etc/config/system.glGammaVal (the
default value being 1.70; applications running on Irix 6.2 or
above may simply pass the sRGB ICC profile to the color
management system)
NeXT using NeXTStep
apply gamma 2.22
"Applying gamma" means that each of the three R, G and B must be
converted to R'=R^gamma, G'=G^gamma, B'=B^gamma, before being handed
to the OS.
This may be done rapidly by building a 256-element lookup table once
per browser invocation thus:
for i := 0 to 255 do
raw := i / 255.0;
corr := pow (raw, gamma);
table[i] := trunc (0.5 + corr * 255.0)
end
which then avoids any need to do transcendental math per color
attribute, far less per pixel.
_________________________________________________________________
_________________________________________________________________
15 Fonts
Contents
* 15.1 Introduction
* 15.2 Font matching algorithm
* 15.3 Font family: the 'font-family' property
+ 15.3.1 Generic font families
o 15.3.1.1 serif
o 15.3.1.2 sans-serif
o 15.3.1.3 cursive
o 15.3.1.4 fantasy
o 15.3.1.5 monospace
* 15.4 Font styling: the 'font-style' property
* 15.5 Small-caps: the 'font-variant' property
* 15.6 Font boldness: the 'font-weight' property
* 15.7 Font size: the 'font-size' property
* 15.8 Shorthand font property: the 'font' property
15.1 Introduction
Setting font properties will be among the most common uses of style
sheets. Unfortunately, there exists no well-defined and universally
accepted taxonomy for classifying fonts, and terms that apply to one
font family may not be appropriate for others. E.g. 'italic' is
commonly used to label slanted text, but slanted text may also be
labeled as being Oblique, Slanted, Incline, Cursive or Kursiv.
Therefore it is not a simple problem to map typical font selection
properties to a specific font.
15.2 Font matching algorithm
Because there is no accepted, universal taxonomy of font properties,
matching of properties to font faces must be done carefully. The
properties are matched in a well-defined order to insure that the
results of this matching process are as consistent as possible across
UAs (assuming that the same library of font faces is presented to each
of them).
1. The User Agent makes (or accesses) a database of relevant CSS 2.1
properties of all the fonts of which the UA is aware. If there are
two fonts with exactly the same properties, the user agent selects
one of them.
2. At a given element and for each character in that element, the UA
assembles the font properties applicable to that element. Using
the complete set of properties, the UA uses the 'font-family'
property to choose a tentative font family. The remaining
properties are tested against the family according to the matching
criteria described with each property. If there are matches for
all the remaining properties, then that is the matching font face
for the given element or character.
3. If there is no matching font face within the 'font-family' being
processed by step 2, and if there is a next alternative
'font-family' in the font set, then repeat step 2 with the next
alternative 'font-family'.
4. If there is a matching font face, but it doesn't contain a glyph
for the current character, and if there is a next alternative
'font-family' in the font sets, then repeat step 2 with the next
alternative 'font-family'.
5. If there is no font within the family selected in 2, then use a
UA-dependent default 'font-family' and repeat step 2, using the
best match that can be obtained within the default font. If a
particular character cannot be displayed using this font, then the
UA may use other means to determine a suitable font for that
character. The UA should map each character for which it has no
suitable font to a visible symbol chosen by the UA, preferably a
"missing character" glyph from one of the font faces available to
the UA.
(The above algorithm can be optimized to avoid having to revisit the
CSS 2.1 properties for each character.)
The per-property matching rules from (2) above are as follows:
1. 'font-style' is tried first. 'italic' will be satisfied if there
is either a face in the UA's font database labeled with the CSS
keyword 'italic' (preferred) or 'oblique'. Otherwise the values
must be matched exactly or font-style will fail.
2. 'font-variant' is tried next. 'small-caps' matches (1) a font
labeled as 'small-caps', (2) a font in which the small caps are
synthesized, or (3) a font where all lowercase letters are
replaced by upper case letters. A small-caps font may be
synthesized by electronically scaling uppercase letters from a
normal font. 'normal' matches a font's normal (non-small-caps)
variant. A font cannot fail to have a normal variant. A font that
is only available as small-caps shall be selectable as either a
'normal' face or a 'small-caps' face.
3. 'font-weight' is matched next, it will never fail. (See
'font-weight' below.)
4. 'font-size' must be matched within a UA-dependent margin of
tolerance. (Typically, sizes for scalable fonts are rounded to the
nearest whole pixel, while the tolerance for bitmapped fonts could
be as large as 20%.) Further computations, e.g. by 'em' values in
other properties, are based on the computed value of 'font-size'.
15.3 Font family: the 'font-family' property
'font-family'
Value: [[ | ] [, |
]* ] | inherit
Initial: depends on user agent
Applies to: all elements
Inherited: yes
Percentages: N/A
Media: visual
Computed value: as specified
The value is a prioritized list of font family names and/or generic
family names. Unlike most other CSS properties, values are separated
by a comma to indicate that they are alternatives:
body { font-family: Gill, Helvetica, sans-serif }
Although many fonts provide the "missing character" glyph, typically
an open box, as its name implies this should not be considered a match
for characters that cannot be found in the font. (It should, however,
be considered a match for U+FFFD, the "missing character" character's
code point).
There are two types of font family names:
The name of a font family of choice. In the last example,
"Gill" and "Helvetica" are font families.
In the example above, the last value is a generic family name.
The following generic families are defined:
+ 'serif' (e.g. Times)
+ 'sans-serif' (e.g. Helvetica)
+ 'cursive' (e.g. Zapf-Chancery)
+ 'fantasy' (e.g. Western)
+ 'monospace' (e.g. Courier)
Style sheet designers are encouraged to offer a generic font
family as a last alternative. Generic font family names are
keywords and must NOT be quoted.
If an unquoted font family name contains parentheses, brackets, and/or
braces, they must still be escaped per CSS grammar rules. Similarly,
quotation marks (both single and double), semicolons, exclamation
marks, commas, and leading slashes within unquoted font family names
must be escaped. Font names containing any such characters or
whitespace should be quoted:
body { font-family: "New Century Schoolbook", serif }
If quoting is omitted, any whitespace characters before and after the
font name are ignored and any sequence of whitespace characters inside
the font name is converted to a single space. Font family names that
happen to be the same as a keyword value (e.g. 'initial', 'inherit',
'default', 'serif', 'sans-serif', 'monospace', 'fantasy', and
'cursive') must be quoted to prevent confusion with the keywords with
the same names. UAs must not consider these keywords as matching the
'' type.
15.3.1 Generic font families
Generic font families are a fallback mechanism, a means of preserving
some of the style sheet author's intent in the worst case when none of
the specified fonts can be selected. For optimum typographic control,
particular named fonts should be used in style sheets.
All five generic font families are defined to exist in all CSS
implementations (they need not necessarily map to five distinct actual
fonts). User agents should provide reasonable default choices for the
generic font families, which express the characteristics of each
family as well as possible within the limits allowed by the underlying
technology.
User agents are encouraged to allow users to select alternative
choices for the generic fonts.
15.3.1.1 serif
Glyphs of serif fonts, as the term is used in CSS, have finishing
strokes, flared or tapering ends, or have actual serifed endings
(including slab serifs). Serif fonts are typically
proportionately-spaced. They often display a greater variation between
thick and thin strokes than fonts from the 'sans-serif' generic font
family. CSS uses the term 'serif' to apply to a font for any script,
although other names may be more familiar for particular scripts, such
as Mincho (Japanese), Sung or Song (Chinese), Totum or Kodig (Korean).
Any font that is so described may be used to represent the generic
'serif' family.
Examples of fonts that fit this description include:
Latin fonts Times New Roman, Bodoni, Garamond, Minion Web, ITC Stone
Serif, MS Georgia, Bitstream Cyberbit
Greek fonts Bitstream Cyberbit
Cyrillic fonts Adobe Minion Cyrillic, Excelcior Cyrillic Upright,
Monotype Albion 70, Bitstream Cyberbit, ER Bukinst
Hebrew fonts New Peninim, Raanana, Bitstream Cyberbit
Japanese fonts Ryumin Light-KL, Kyokasho ICA, Futo Min A101
Arabic fonts Bitstream Cyberbit
Cherokee fonts Lo Cicero Cherokee
15.3.1.2 sans-serif
Glyphs in sans-serif fonts, as the term is used in CSS, have stroke
endings that are plain -- without any flaring, cross stroke, or other
ornamentation. Sans-serif fonts are typically proportionately-spaced.
They often have little variation between thick and thin strokes,
compared to fonts from the 'serif' family. CSS uses the term
'sans-serif' to apply to a font for any script, although other names
may be more familiar for particular scripts, such as Gothic
(Japanese), Kai (Chinese), or Pathang (Korean). Any font that is so
described may be used to represent the generic 'sans-serif' family.
Examples of fonts that fit this description include:
Latin fonts MS Trebuchet, ITC Avant Garde Gothic, MS Arial, MS
Verdana, Univers, Futura, ITC Stone Sans, Gill Sans, Akzidenz Grotesk,
Helvetica
Greek fonts Attika, Typiko New Era, MS Tahoma, Monotype Gill Sans 571,
Helvetica Greek
Cyrillic fonts Helvetica Cyrillic, ER Univers, Lucida Sans Unicode,
Bastion
Hebrew fonts Arial Hebrew, MS Tahoma
Japanese fonts Shin Go, Heisei Kaku Gothic W5
Arabic fonts MS Tahoma
15.3.1.3 cursive
Glyphs in cursive fonts, as the term is used in CSS, generally have
either joining strokes or other cursive characteristics beyond those
of italic typefaces. The glyphs are partially or completely connected,
and the result looks more like handwritten pen or brush writing than
printed letterwork. Fonts for some scripts, such as Arabic, are almost
always cursive. CSS uses the term 'cursive' to apply to a font for any
script, although other names such as Chancery, Brush, Swing and Script
are also used in font names.
Examples of fonts that fit this description include:
Latin fonts Caflisch Script, Adobe Poetica, Sanvito, Ex Ponto, Snell
Roundhand, Zapf-Chancery
Cyrillic fonts ER Architekt
Hebrew fonts Corsiva
Arabic fonts DecoType Naskh, Monotype Urdu 507
15.3.1.4 fantasy
Fantasy fonts, as used in CSS, are primarily decorative while still
containing representations of characters (as opposed to Pi or Picture
fonts, which do not represent characters). Examples include:
Latin fonts Alpha Geometrique, Critter, Cottonwood, FB Reactor, Studz
15.3.1.5 monospace
The sole criterion of a monospace font is that all glyphs have the
same fixed width. (This can make some scripts, such as Arabic, look
most peculiar.) The effect is similar to a manual typewriter, and is
often used to set samples of computer code.
Examples of fonts which fit this description include:
Latin fonts Courier, MS Courier New, Prestige, Everson Mono
Greek Fonts MS Courier New, Everson Mono
Cyrillic fonts ER Kurier, Everson Mono
Japanese fonts Osaka Monospaced
Cherokee fonts Everson Mono
15.4 Font styling: the 'font-style' property
'font-style'
Value: normal | italic | oblique | inherit
Initial: normal
Applies to: all elements
Inherited: yes
Percentages: N/A
Media: visual
Computed value: as specified
The 'font-style' property selects between normal (sometimes referred
to as "roman" or "upright"), italic and oblique faces within a font
family.
A value of 'normal' selects a font that is classified as 'normal' in
the UA's font database, while 'oblique' selects a font that is labeled
'oblique'. A value of 'italic' selects a font that is labeled
'italic', or, if that is not available, one labeled 'oblique'.
The font that is labeled 'oblique' in the UA's font database may
actually have been generated by electronically slanting a normal font.
Fonts with Oblique, Slanted or Incline in their names will typically
be labeled 'oblique' in the UA's font database. Fonts with Italic,
Cursive or Kursiv in their names will typically be labeled 'italic'.
h1, h2, h3 { font-style: italic }
h1 em { font-style: normal }
In the example above, emphasized text within 'H1' will appear in a
normal face.
15.5 Small-caps: the 'font-variant' property
'font-variant'
Value: normal | small-caps | inherit
Initial: normal
Applies to: all elements
Inherited: yes
Percentages: N/A
Media: visual
Computed value: as specified
Another type of variation within a font family is the small-caps. In a
small-caps font the lower case letters look similar to the uppercase
ones, but in a smaller size and with slightly different proportions.
The 'font-variant' property selects that font.
A value of 'normal' selects a font that is not a small-caps font,
'small-caps' selects a small-caps font. It is acceptable (but not
required) in CSS 2.1 if the small-caps font is a created by taking a
normal font and replacing the lower case letters by scaled uppercase
characters. As a last resort, uppercase letters will be used as
replacement for a small-caps font.
The following example results in an 'H3' element in small-caps, with
any emphasized words in oblique, and any emphasized words within an
'H3' oblique small-caps:
h3 { font-variant: small-caps }
em { font-style: oblique }
There may be other variants in the font family as well, such as fonts
with old-style numerals, small-caps numerals, condensed or expanded
letters, etc. CSS 2.1 has no properties that select those.
Note: insofar as this property causes text to be transformed to
uppercase, the same considerations as for 'text-transform' apply.
15.6 Font boldness: the 'font-weight' property
'font-weight'
Value: normal | bold | bolder | lighter | 100 | 200 | 300 | 400 |
500 | 600 | 700 | 800 | 900 | inherit
Initial: normal
Applies to: all elements
Inherited: yes
Percentages: N/A
Media: visual
Computed value: see text
The 'font-weight' property selects the weight of the font. The values
'100' to '900' form an ordered sequence, where each number indicates a
weight that is at least as dark as its predecessor. The keyword
'normal' is synonymous with '400', and 'bold' is synonymous with
'700'. Keywords other than 'normal' and 'bold' have been shown to be
often confused with font names and a numerical scale was therefore
chosen for the 9-value list.
p { font-weight: normal } /* 400 */
h1 { font-weight: 700 } /* bold */
The 'bolder' and 'lighter' values select font weights that are
relative to the weight inherited from the parent:
strong { font-weight: bolder }
Fonts (the font data) typically have one or more properties whose
values are names that are descriptive of the "weight" of a font. There
is no accepted, universal meaning to these weight names. Their primary
role is to distinguish faces of differing darkness within a single
font family. Usage across font families is quite variant; for example,
a font that one might think of as being bold might be described as
being Regular, Roman, Book, Medium, Semi- or DemiBold, Bold, or Black,
depending on how black the "normal" face of the font is within the
design. Because there is no standard usage of names, the weight
property values in CSS 2.1 are given on a numerical scale in which the
value '400' (or 'normal') corresponds to the "normal" text face for
that family. The weight name associated with that face will typically
be Book, Regular, Roman, Normal or sometimes Medium.
The association of other weights within a family to the numerical
weight values is intended only to preserve the ordering of darkness
within that family. However, the following heuristics tell how the
assignment is done in typical cases:
* If the font family already uses a numerical scale with nine values
(like e.g. OpenType does), the font weights should be mapped
directly.
* If there is both a face labeled Medium and one labeled Book,
Regular, Roman or Normal, then the Medium is normally assigned to
the '500'.
* The font labeled "Bold" will often correspond to the weight value
'700'.
* If there are fewer then 9 weights in the family, the default
algorithm for filling the "holes" is as follows. If '500' is
unassigned, it will be assigned the same font as '400'. If any of
the values '600', '700', '800' or '900' remains unassigned, they
are assigned to the same face as the next darker assigned keyword,
if any, or the next lighter one otherwise. If any of '300', '200'
or '100' remains unassigned, it is assigned to the next lighter
assigned keyword, if any, or the next darker otherwise.
The following two examples show typical mappings.
Assume four weights in the "Rattlesnake" family, from lightest to
darkest: Regular, Medium, Bold, Heavy.
CAPTION: First example of font-weight mapping
Available faces Assignments Filling the holes
"Rattlesnake Regular" 400 100, 200, 300
"Rattlesnake Medium" 500
"Rattlesnake Bold" 700 600
"Rattlesnake Heavy" 800 900
Assume six weights in the "Ice Prawn" family: Book, Medium, Bold,
Heavy, Black, ExtraBlack. Note that in this instance the user agent
has decided not to assign a numeric value to "Ice Prawn ExtraBlack".
CAPTION: Second example of font-weight mapping
Available faces Assignments Filling the holes
"Ice Prawn Book" 400 100, 200, 300
"Ice Prawn Medium" 500
"Ice Prawn Bold" 700 600
"Ice Prawn Heavy" 800
"Ice Prawn Black" 900
"Ice Prawn ExtraBlack" (none)
Since the intent of the relative keywords 'bolder' and 'lighter' is to
darken or lighten the face within the family and because a family may
not have faces aligned with all the symbolic weight values, the
matching of 'bolder' is to the next darker face available on the
client within the family and the matching of 'lighter' is to the next
lighter face within the family. To be precise, the meaning of the
relative keywords 'bolder' and 'lighter' is as follows:
* 'bolder' selects the next weight that is assigned to a font that
is darker than the inherited one. If there is no such weight, it
simply results in the next darker numerical value (and the font
remains unchanged), unless the inherited value was '900' in which
case the resulting weight is also '900'.
* 'lighter' is similar, but works in the opposite direction: it
selects the next lighter keyword with a different font from the
inherited one, unless there is no such font, in which case it
selects the next lighter numerical value (and keeps the font
unchanged).
There is no guarantee that there will be a darker face for each of the
'font-weight' values; for example, some fonts may have only a normal
and a bold face, while others may have eight face weights. There is no
guarantee on how a UA will map font faces within a family to weight
values. The only guarantee is that a face of a given value will be no
less dark than the faces of lighter values.
The computed value of "font-weight" is either:
* one of the legal number values, or
* one of the legal number values combined with one or more of the
relative values (bolder or lighter). This type of computed values
is necessary to use when the font in question does not have all
weight variations that are needed.
CSS 2.1 does not specify how the computed value of font-weight is
represented internally or externally.
15.7 Font size: the 'font-size' property
'font-size'
Value: | | | |
inherit
Initial: medium
Applies to: all elements
Inherited: yes
Percentages: refer to parent element's font size
Media: visual
Computed value: absolute length
The font size corresponds to the em square, a concept used in
typography. Note that certain glyphs may bleed outside their em
squares. Values have the following meanings:
An keyword is an index to a table of font sizes
computed and kept by the UA. Possible values are:
[ xx-small | x-small | small | medium | large | x-large |
xx-large ]
The following table provides user agent guidelines for the
absolute-size mapping to HTML heading and absolute font-sizes.
The 'medium' value is the user's preferred font size and is
used as the reference middle value.
CSS absolute-size values xx-small x-small small medium large x-large
xx-large
HTML font sizes 1 2 3 4 5 6 7
Note: implementation experience has demonstrated that a fixed
ratio between adjacent absolute-size keywords is problematic,
and this specification does NOT recommend such a fixed ratio,
in contrast to previous specifications (CSS1 suggested 1.5, and
CSS2 suggested 1.2).
Implementors should build a table of scaling factors for
absolute-size keywords relative to the 'medium' font size and
the particular device and its characteristics (e.g. the
resolution of the device).
Different media may need different scaling factors. Also, the
UA should take the quality and availability of fonts into
account when computing the table. The table may be different
from one font family to another.
Note 1. To preserve readability, a UA applying these guidelines
should nevertheless avoid creating font-size resulting in less
than 9 pixels per EM unit on a computer display.
Note 2. In CSS1, the suggested scaling factor between adjacent
indexes was 1.5 which user experience proved to be too large.
In CSS2, the suggested scaling factor for computer screen
between adjacent indexes was 1.2 which still created issues for
the small sizes. The new scaling factor varies between each
index to provide better readability.
A keyword is interpreted relative to the table
of font sizes and the font size of the parent element. Possible
values are: [ larger | smaller ]. For example, if the parent
element has a font size of 'medium', a value of 'larger' will
make the font size of the current element be 'large'. If the
parent element's size is not close to a table entry, the UA is
free to interpolate between table entries or round off to the
closest one. The UA may have to extrapolate table values if the
numerical value goes beyond the keywords.
Length and percentage values should not take the font size table into
account when calculating the font size of the element.
Negative values are not allowed.
On all other properties, 'em' and 'ex' length values refer to the
computed font size of the current element. On the 'font-size'
property, these length units refer to the computed font size of the
parent element.
Note that an application may reinterpret an explicit size, depending
on the context. E.g., inside a VR scene a font may get a different
size because of perspective distortion.
Examples:
p { font-size: 16px; }
@media print {
p { font-size: 12pt; }
}
blockquote { font-size: larger }
em { font-size: 150% }
em { font-size: 1.5em }
15.8 Shorthand font property: the 'font' property
'font'
Value: [ [ || || ]?
[ / ]? ] | caption |
icon | menu | message-box | small-caption | status-bar | inherit
Initial: see individual properties
Applies to: all elements
Inherited: yes
Percentages: see individual properties
Media: visual
Computed value: see individual properties
The 'font' property is, except as described below, a shorthand
property for setting 'font-style', 'font-variant', 'font-weight',
'font-size', 'line-height' and 'font-family' at the same place in the
style sheet. The syntax of this property is based on a traditional
typographical shorthand notation to set multiple properties related to
fonts.
All font-related properties are first reset to their initial values,
including those listed in the preceding paragraph. Then, those
properties that are given explicit values in the 'font' shorthand are
set to those values. For a definition of allowed and initial values,
see the previously defined properties.
p { font: 12px/14px sans-serif }
p { font: 80% sans-serif }
p { font: x-large/110% "New Century Schoolbook", serif }
p { font: bold italic large Palatino, serif }
p { font: normal small-caps 120%/120% fantasy }
In the second rule, the font size percentage value ('80%') refers to
the font size of the parent element. In the third rule, the line
height percentage refers to the font size of the element itself.
In the first three rules above, the 'font-style', 'font-variant' and
'font-weight' are not explicitly mentioned, which means they are all
three set to their initial value ('normal'). The fourth rule sets the
'font-weight' to 'bold', the 'font-style' to 'italic' and implicitly
sets 'font-variant' to 'normal'.
The fifth rule sets the 'font-variant' ('small-caps'), the 'font-size'
(120% of the parent's font), the 'line-height' (120% times the font
size) and the 'font-family' ('fantasy'). It follows that the keyword
'normal' applies to the two remaining properties: 'font-style' and
'font-weight'.
The following values refer to system fonts:
caption
The font used for captioned controls (e.g., buttons,
drop-downs, etc.).
icon
The font used to label icons.
menu
The font used in menus (e.g., dropdown menus and menu lists).
message-box
The font used in dialog boxes.
small-caption
The font used for labeling small controls.
status-bar
The font used in window status bars.
System fonts may only be set as a whole; that is, the font family,
size, weight, style, etc. are all set at the same time. These values
may then be altered individually if desired. If no font with the
indicated characteristics exists on a given platform, the user agent
should either intelligently substitute (e.g., a smaller version of the
'caption' font might be used for the 'small-caption' font), or
substitute a user agent default font. As for regular fonts, if, for a
system font, any of the individual properties are not part of the
operating system's available user preferences, those properties should
be set to their initial values.
That is why this property is "almost" a shorthand property: system
fonts can only be specified with this property, not with 'font-family'
itself, so 'font' allows authors to do more than the sum of its
subproperties. However, the individual properties such as
'font-weight' are still given values taken from the system font, which
can be independently varied.
Example(s):
button { font: 300 italic 1.3em/1.7em "FB Armada", sans-serif }
button p { font: menu }
button p em { font-weight: bolder }
If the font used for dropdown menus on a particular system happened to
be, for example, 9-point Charcoal, with a weight of 600, then P
elements that were descendants of BUTTON would be displayed as if this
rule were in effect:
button p { font: 600 9px Charcoal }
Because the 'font' shorthand property resets any property not
explicitly given a value to its initial value, this has the same
effect as this declaration:
button p {
font-family: Charcoal;
font-style: normal;
font-variant: normal;
font-weight: 600;
font-size: 9px;
line-height: normal;
}
_________________________________________________________________
_________________________________________________________________
16 Text
Contents
* 16.1 Indentation: the 'text-indent' property
* 16.2 Alignment: the 'text-align' property
* 16.3 Decoration
+ 16.3.1 Underlining, overlining, striking, and blinking: the
'text-decoration' property
* 16.4 Letter and word spacing: the 'letter-spacing' and
'word-spacing' properties
* 16.5 Capitalization: the 'text-transform' property
* 16.6 Whitespace: the 'white-space' property
+ 16.6.1 The 'white-space' processing model
+ 16.6.2 Example of bidirectionality with white-space
collapsing
+ 16.6.3 Control and combining characters' details
The properties defined in the following sections affect the visual
presentation of characters, spaces, words, and paragraphs.
16.1 Indentation: the 'text-indent' property
'text-indent'
Value: | | inherit
Initial: 0
Applies to: block-level elements, table cells and inline blocks
Inherited: yes
Percentages: refer to width of containing block
Media: visual
Computed value: the percentage as specified or the absolute length
This property specifies the indentation of the first line of text in a
block. More precisely, it specifies the indentation of the first box
that flows into the block's first line box. The box is indented with
respect to the left (or right, for right-to-left layout) edge of the
line box. User agents should render this indentation as blank space.
Values have the following meanings:
The indentation is a fixed length.
The indentation is a percentage of the containing block width.
The value of 'text-indent' may be negative, but there may be
implementation-specific limits. If the value of 'text-indent' is
either negative or exceeds the width of the block, that first box,
described above, can overflow the block. The value of 'overflow' will
affect whether such text that overflows the block is visible.
Example(s):
The following example causes a '3em' text indent.
p { text-indent: 3em }
Note: Since the 'text-indent' property inherits, when specified on a
block element, it will affect descendent inline-block elements. For
this reason, it is often wise to specify 'text-indent: 0' on elements
that are specified 'display:inline-block'.
16.2 Alignment: the 'text-align' property
'text-align'
Value: left | right | center | justify | inherit
Initial: a nameless value that acts as 'left' if 'direction' is
'ltr', 'right' if 'direction' is 'rtl'
Applies to: block-level elements, table cells and inline blocks
Inherited: yes
Percentages: N/A
Media: visual
Computed value: the initial value or as specified
This property describes how inline content of a block is aligned.
Values have the following meanings:
left, right, center, justify
Left, right, center, and justify text, respectively, as
described in the section on inline formatting.
A block of text is a stack of line boxes. In the case of 'left',
'right' and 'center', this property specifies how the inline boxes
within each line box align with respect to the line box's left and
right sides; alignment is not with respect to the viewport. In the
case of 'justify', this property specifies that the inline boxes are
to be made flush with both sides of the block. (See also
'letter-spacing' and 'word-spacing'.)
If the computed value of text-align is 'justify' while the computed
value of white-space is 'pre' or 'pre-line', the actual value of
text-align is set to the initial value.
Example(s):
In this example, note that since 'text-align' is inherited, all
block-level elements inside DIV elements with a class name of
'important' will have their inline content centered.
div.important { text-align: center }
Note. The actual justification algorithm used depends on the
user-agent and the language/script of the text.
Conforming user agents may interpret the value 'justify' as 'left' or
'right', depending on whether the element's default writing direction
is left-to-right or right-to-left, respectively.
16.3 Decoration
16.3.1 Underlining, overlining, striking, and blinking: the 'text-decoration'
property
'text-decoration'
Value: none | [ underline || overline || line-through || blink ] |
inherit
Initial: none
Applies to: all elements
Inherited: no (see prose)
Percentages: N/A
Media: visual
Computed value: as specified
This property describes decorations that are added to the text of an
element using the element's color. When specified on an inline
element, it affects all the boxes generated by that element; for all
other elements, the decorations are propagated to an anonymous inline
box that wraps all the in-flow inline children of the element, and to
any block-level in-flow descendants. It is not, however, further
propagated to floating and absolutely positioned descendants, nor to
the contents of 'inline-table' and 'inline-block' descendants.
Underlines, overlines, and line-throughs are applied only to text
(including white space, letter spacing, and word spacing): margins,
borders, and padding are skipped. If an element contains no text, user
agents must refrain from rendering these text decorations on the
element. For example, images will not be underlined.
The 'text-decoration' property on descendant elements cannot have any
effect on the decoration of the ancestor. In determining the position
of and thickness of text decoration lines, user agents may consider
the font sizes of and dominant baselines of descendants, but must use
the same baseline and thickness on each line. Relatively positioning a
descendant moves all text decorations affecting it along with the
descendant's text; it does not affect calculation of the decoration's
initial position on that line.
Values have the following meanings:
none
Produces no text decoration.
underline
Each line of text is underlined.
overline
Each line of text has a line above it.
line-through
Each line of text has a line through the middle.
blink
Text blinks (alternates between visible and invisible).
Conforming user agents may simply not blink the text. Note that
not blinking the text is one technique to satisfy checkpoint
3.3 of WAI-UAAG.
The color(s) required for the text decoration must be derived from the
'color' property value of the element on which 'text-decoration' is
set. The color of decorations must remain the same even if descendant
elements have different 'color' values.
Some user agents have implemented text-decoration by propagating the
decoration to the descendant elements as opposed to preserving a
constant thickness and line position as described above. This was
arguably allowed by the looser wording in CSS2. SVG1, CSS1-only, and
CSS2-only user agents may implement the older model and still claim
conformance to this part of CSS 2.1. (This does not apply to UAs
developed after this specification was released.)
Example(s):
In the following example for HTML, the text content of all A elements
acting as hyperlinks (whether visited or not) will be underlined:
a:visited,a:link { text-decoration: underline }
Example(s):
In the following style sheet and document fragment:
blockquote { text-decoration: underline; color: blue; }
em { display: block; }
cite { color: fuchsia; }

Help, help!
I am under a hat! --GwieF

...the underlining for the blockquote element is propagated to an
anonymous inline element that surrounds the span element, causing the
text "Help, help!" to be blue, with the blue underlining from the
anonymous inline underneath it, the color being taken from the
blockquote element. The text in the em block is also
underlined, as it is in an in-flow block to which the underline is
propagated. The final line of text is fuchsia, but the underline
underneath it is still the blue underline from the anonymous inline
element.
Sample rendering of the above underline example
This diagram shows the boxes involved in the example above. The
rounded aqua line represents the anonymous inline element wrapping the
inline contents of the paragraph element, the rounded blue line
represents the span element, and the orange lines represent the
blocks.
16.4 Letter and word spacing: the 'letter-spacing' and 'word-spacing'
properties
'letter-spacing'
Value: normal | | inherit
Initial: normal
Applies to: all elements
Inherited: yes
Percentages: N/A
Media: visual
Computed value: 'normal' or absolute length
This property specifies spacing behavior between text characters.
Values have the following meanings:
normal
The spacing is the normal spacing for the current font. This
value allows the user agent to alter the space between
characters in order to justify text.
This value indicates inter-character space in addition to the
default space between characters. Values may be negative, but
there may be implementation-specific limits. User agents may
not further increase or decrease the inter-character space in
order to justify text.
Character spacing algorithms are user agent-dependent.
Example(s):
In this example, the space between characters in BLOCKQUOTE elements
is increased by '0.1em'.
blockquote { letter-spacing: 0.1em }
In the following example, the user agent is not permitted to alter
inter-character space:
blockquote { letter-spacing: 0cm } /* Same as '0' */
When the resultant space between two characters is not the same as the
default space, user agents should not use ligatures.
'word-spacing'
Value: normal | | inherit
Initial: normal
Applies to: all elements
Inherited: yes
Percentages: N/A
Media: visual
Computed value: for 'normal' the value '0'; otherwise the absolute
length
This property specifies spacing behavior between words. Values have
the following meanings:
normal
The normal inter-word space, as defined by the current font
and/or the UA.
This value indicates inter-word space in addition to the
default space between words. Values may be negative, but there
may be implementation-specific limits.
Word spacing algorithms are user agent-dependent. Word spacing is also
influenced by justification (see the 'text-align' property). Word
spacing affects each space (U+0020), non-breaking space (U+00A0), and
ideographic space (U+3000) left in the text after the white space
processing rules have been applied.
Example(s):
In this example, the word-spacing between each word in H1 elements is
increased by '1em'.
h1 { word-spacing: 1em }
16.5 Capitalization: the 'text-transform' property
'text-transform'
Value: capitalize | uppercase | lowercase | none | inherit
Initial: none
Applies to: all elements
Inherited: yes
Percentages: N/A
Media: visual
Computed value: as specified
This property controls capitalization effects of an element's text.
Values have the following meanings:
capitalize
Puts the first character of each word in uppercase; other
characters are unaffected.
uppercase
Puts all characters of each word in uppercase.
lowercase
Puts all characters of each word in lowercase.
none
No capitalization effects.
The actual transformation in each case is written language dependent.
See RFC 3066 ([RFC3066]) for ways to find the language of an element.
Only characters belonging to "bicameral scripts" [UNICODE] are
affected.
Example(s):
In this example, all text in an H1 element is transformed to uppercase
text.
h1 { text-transform: uppercase }
16.6 Whitespace: the 'white-space' property
'white-space'
Value: normal | pre | nowrap | pre-wrap | pre-line | inherit
Initial: normal
Applies to: all elements
Inherited: yes
Percentages: N/A
Media: visual
Computed value: as specified
This property declares how whitespace inside the element is handled.
Values have the following meanings:
normal
This value directs user agents to collapse sequences of
whitespace, and break lines as necessary to fill line boxes.
pre
This value prevents user agents from collapsing sequences of
whitespace. Lines are only broken at newlines in the source, or
at occurrences of "\A" in generated content.
nowrap
This value collapses whitespace as for 'normal', but suppresses
line breaks within text.
pre-wrap
This value prevents user agents from collapsing sequences of
whitespace. Lines are broken at newlines in the source, at
occurrences of "\A" in generated content, and as necessary to
fill line boxes.
pre-line
This value directs user agents to collapse sequences of
whitespace. Lines are broken at newlines in the source, at
occurrences of "\A" in generated content, and as necessary to
fill line boxes.
Newlines in the source can be represented by a carriage return
(U+000D), a linefeed (U+000A) or both (U+000D U+000A) or by some other
mechanism that identifies the beginning and end of document segments,
such as the SGML RECORD-START and RECORD-END tokens. The CSS
'white-space' processing model assumes all newlines have been
normalized to line feeds.
Example(s):
The following examples show what whitespace behavior is expected from
the PRE and P elements, the "nowrap" attribute in HTML, and in
generated content.
pre { white-space: pre }
p { white-space: normal }
td[nowrap] { white-space: nowrap }
:before,:after { white-space: pre-line }
In addition, the effect of an HTML PRE element with the non-standard
"wrap" attribute is demonstrated by the following example:
pre[wrap] { white-space: pre-wrap }
16.6.1 The 'white-space' processing model
Any text that is directly contained inside a block element (not inside
an inline element) should be treated as an anonymous inline element.
For each inline element (including anonymous inlines elements), the
following steps are performed, treating bidi formatting characters as
if they were not there:
1. Each tab (U+0009), carriage return (U+000D), or space (U+0020)
character surrounding a linefeed (U+000A) character is removed if
'white-space' is set to 'normal', 'nowrap', or 'pre-line'.
2. If 'white-space' is set to 'pre' or 'pre-wrap', any sequence of
spaces (U+0020) unbroken by an element boundary is treated as a
sequence of non-breaking spaces. However, for 'pre-wrap', a line
breaking opportunity exists at the end of the sequence.
3. If 'white-space' is set to 'normal' or 'nowrap', linefeed
characters are transformed for rendering purpose into one of the
following characters: a space character, a zero width space
character (U+200B), or no character (i.e. not rendered), according
to UA-specific algorithms based on the content script.
4. If 'white-space' is set to 'normal', 'nowrap', or 'pre-line',
1. every tab (U+0009) is converted to a space (U+0020)
2. any space (U+0020) following another space (U+0020) -- even a
space before the inline, if that space also has 'white-space'
set to 'normal', 'nowrap' or 'pre-line' -- is removed.
Then, the entire block is rendered. Inlines are laid out, taking bidi
reordering into account, and wrapping as specified by the
'white-space' property.
As each line is laid out,
1. If a space (U+0020) at the beginning of a line has 'white-space'
set to 'normal', 'nowrap', or 'pre-line', it is removed.
2. All tabs (U+0009) are rendered as a horizontal shift that lines up
the start edge of the next glyph with the next tab stop. Tab stops
occur at points that are mutiples of 8 times the width of a space
(U+0020) rendered in the block's font from the block's starting
content edge.
3. If a space (U+0020) at the end of a line has 'white-space' set to
'normal', 'nowrap', or 'pre-line', it is also removed.
4. If spaces (U+0020) or tabs (U+0009) at the end of a line have
'white-space' set to 'pre-wrap', UAs may visually collapse them.
A float should not introduce a line break opportunity.
Note. CSS 2.1 does not fully define where line breaking opportunities
occur. Floated and absolutely-positioned elements do not introduce a
line breaking opportunity.
16.6.2 Example of bidirectionality with white-space collapsing
Given the following markup fragment, taking special note of spaces
(with varied backgrounds and borders for emphasis and identification):
A B C
...where the element represents a left-to-right embedding and
the element represents a right-to-left embedding, and assuming
that the 'white-space' property is set to 'normal', the above
processing model would result in the following:
* The space before the B ( ) would collapse with the space after the
A ( ).
* The space before the C ( ) would collapse with the space after the
B ( ).
This would leave two spaces, one after the A in the left-to-right
embedding level, and one after the B in the right-to-left embedding
level. This is then rendered according to the Unicode bidirectional
algorithm, with the end result being:
A BC
Note that there are two spaces between A and B, and none between B and
C. This can sometimes be avoided by using the natural bidirectionality
of characters instead of explicit embedding levels. Also, it is good
to avoid spaces immediately inside start and end tags, as these tend
to do weird things when dealing with white space collapsing.
16.6.3 Control and combining characters' details
Control characters other than U+0009 (tab), U+000A (line feed), U+0020
(space), and U+202x (bidi formatting characters) are treated as
characters to render in the same way as any normal character.
Combining characters should be treated as part of the character with
which they are supposed to combine. For example, :first-letter styles
the entire glyph if you have content like "ö"; it
doesn't just match the base character.
_________________________________________________________________
_________________________________________________________________
17 Tables
Contents
* 17.1 Introduction to tables
* 17.2 The CSS table model
+ 17.2.1 Anonymous table objects
* 17.3 Columns
* 17.4 Tables in the visual formatting model
+ 17.4.1 Caption position and alignment
* 17.5 Visual layout of table contents
+ 17.5.1 Table layers and transparency
+ 17.5.2 Table width algorithms: the 'table-layout' property
o 17.5.2.1 Fixed table layout
o 17.5.2.2 Automatic table layout
+ 17.5.3 Table height algorithms
+ 17.5.4 Horizontal alignment in a column
+ 17.5.5 Dynamic row and column effects
* 17.6 Borders
+ 17.6.1 The separated borders model
o 17.6.1.1 Borders and Backgrounds around empty cells: the
'empty-cells' property
+ 17.6.2 The collapsing border model
o 17.6.2.1 Border conflict resolution
+ 17.6.3 Border styles
17.1 Introduction to tables
This chapter defines the processing model for tables in CSS. Part of
this processing model is the layout. For the layout, this chapter
introduces two algorithms; the first, the fixed table layout
algorithm, is well-defined, but the second, the automatic table layout
algorithm, is not fully defined by this specification.
For the automatic table layout algorithm, some widely deployed
implementations have achieved relatively close interoperability.
Table layout can be used to represent tabular relationships between
data. Authors specify these relationships in the document language and
can specify their presentation using CSS 2.1.
In a visual medium, CSS tables can also be used to achieve specific
layouts. In this case, authors should not use table-related elements
in the document language, but should apply the CSS to the relevant
structural elements to achieve the desired layout.
Authors may specify the visual formatting of a table as a rectangular
grid of cells. Rows and columns of cells may be organized into row
groups and column groups. Rows, columns, row groups, column groups,
and cells may have borders drawn around them (there are two border
models in CSS 2.1). Authors may align data vertically or horizontally
within a cell and align data in all cells of a row or column.
Example(s):
Here is a simple three-row, three-column table described in HTML 4:

This is a simple 3x3 table

Header 1

Cell 1

Cell 2

Header 2

Cell 3

Cell 4

Header 3

Cell 5

Cell 6

This code creates one table (the TABLE element), three rows (the TR
elements), three header cells (the TH elements), and six data cells
(the TD elements). Note that the three columns of this example are
specified implicitly: there are as many columns in the table as
required by header and data cells.
The following CSS rule centers the text horizontally in the header
cells and presents the text in the header cells with a bold font
weight:
th { text-align: center; font-weight: bold }
The next rules align the text of the header cells on their baseline
and vertically center the text in each data cell:
th { vertical-align: baseline }
td { vertical-align: middle }
The next rules specify that the top row will be surrounded by a 3px
solid blue border and each of the other rows will be surrounded by a
1px solid black border:
table { border-collapse: collapse }
tr#row1 { border: 3px solid blue }
tr#row2 { border: 1px solid black }
tr#row3 { border: 1px solid black }
Note, however, that the borders around the rows overlap where the rows
meet. What color (black or blue) and thickness (1px or 3px) will the
border between row1 and row2 be? We discuss this in the section on
border conflict resolution.
The following rule puts the table caption above the table:
caption { caption-side: top }
The preceding example shows how CSS works with HTML 4 elements; in
HTML 4, the semantics of the various table elements (TABLE, CAPTION,
THEAD, TBODY, TFOOT, COL, COLGROUP, TH, and TD) are well-defined. In
other document languages (such as XML applications), there may not be
pre-defined table elements. Therefore, CSS 2.1 allows authors to "map"
document language elements to table elements via the 'display'
property. For example, the following rule makes the FOO element act
like an HTML TABLE element and the BAR element act like a CAPTION
element:
FOO { display : table }
BAR { display : table-caption }
We discuss the various table elements in the following section. In
this specification, the term table element refers to any element
involved in the creation of a table. An "internal" table element is
one that produces a row, row group, column, column group, or cell.
17.2 The CSS table model
The CSS table model is based on the HTML4 table model, in which the
structure of a table closely parallels the visual layout of the table.
In this model, a table consists of an optional caption and any number
of rows of cells. The table model is said to be "row primary" since
authors specify rows, not columns, explicitly in the document
language. Columns are derived once all the rows have been specified --
the first cell of each row belongs to the first column, the second to
the second column, etc.). Rows and columns may be grouped structurally
and this grouping reflected in presentation (e.g., a border may be
drawn around a group of rows).
Thus, the table model consists of tables, captions, rows, row groups,
columns, column groups, and cells.
The CSS model does not require that the document language include
elements that correspond to each of these components. For document
languages (such as XML applications) that do not have pre-defined
table elements, authors must map document language elements to table
elements; this is done with the 'display' property. The following
'display' values assign table formatting rules to an arbitrary
element:
table (In HTML: TABLE)
Specifies that an element defines a block-level table: it is a
rectangular block that participates in a block formatting
context.
inline-table (In HTML: TABLE)
Specifies that an element defines an inline-level table: it is
a rectangular block that participates in an inline formatting
context).
table-row (In HTML: TR)
Specifies that an element is a row of cells.
table-row-group (In HTML: TBODY)
Specifies that an element groups one or more rows.
table-header-group (In HTML: THEAD)
Like 'table-row-group', but for visual formatting, the row
group is always displayed before all other rows and row groups
and after any top captions. Print user agents may repeat header
rows on each page spanned by a table. If a table contains
multiple elements with 'display: table-header-group', only the
first is rendered as a header; the others are treated as if
they had 'display: table-row-group'.
table-footer-group (In HTML: TFOOT)
Like 'table-row-group', but for visual formatting, the row
group is always displayed after all other rows and row groups
and before any bottom captions. Print user agents may repeat
footer rows on each page spanned by a table. If a table
contains multiple elements with 'display: table-footer-group',
only the first is rendered as a footer; the others are treated
as if they had 'display: table-row-group'.
table-column (In HTML: COL)
Specifies that an element describes a column of cells.
table-column-group (In HTML: COLGROUP)
Specifies that an element groups one or more columns.
table-cell (In HTML: TD, TH)
Specifies that an element represents a table cell.
table-caption (In HTML: CAPTION)
Specifies a caption for the table. All elements with 'display:
table-caption' must be rendered, as described in section 17.4.
Replaced elements with these 'display' values are treated as their
given display types during layout. For example, an image that is set
to 'display: table-cell' will fill the available cell space, and its
dimensions might contribute towards the table sizing algorithms, as
with an ordinary cell.
Elements with 'display' set to 'table-column' or 'table-column-group'
are not rendered (exactly as if they had 'display: none'), but they
are useful, because they may have attributes which induce a certain
style for the columns they represent.
The default style sheet for HTML4 in the appendix illustrates the use
of these values for HTML4:
table { display: table }
tr { display: table-row }
thead { display: table-header-group }
tbody { display: table-row-group }
tfoot { display: table-footer-group }
col { display: table-column }
colgroup { display: table-column-group }
td, th { display: table-cell }
caption { display: table-caption }
User agents may ignore these 'display' property values for HTML table
elements, since HTML tables may be rendered using other algorithms
intended for backwards compatible rendering. However, this is not
meant to discourage the use of 'display: table' on other, non-table
elements in HTML.
17.2.1 Anonymous table objects
Document languages other than HTML may not contain all the elements in
the CSS 2.1 table model. In these cases, the "missing" elements must
be assumed in order for the table model to work. Any table element
will automatically generate necessary anonymous table objects around
itself, consisting of at least three nested objects corresponding to a
'table'/'inline-table' element, a 'table-row' element, and a
'table-cell' element. Missing elements generate anonymous objects
(e.g., anonymous boxes in visual table layout) according to the
following rules:
1. If the parent P of a 'table-cell' box T is not a 'table-row', a
box corresponding to a 'table-row' will be generated between P and
T. This box will span all consecutive 'table-cell' sibling boxes
of T.
2. If the parent P of a 'table-row' box T is not a 'table',
'inline-table', 'table-header-group', 'table-footer-group' or
'table-row-group' box, a box corresponding to a 'table' element
will be generated between P and T. If P is an 'inline' box, then
the generated box must be an 'inline-table' box instead of a
'table' box. This box will span all consecutive sibling boxes of T
that require a 'table' parent: 'table-row', 'table-row-group',
'table-header-group', 'table-footer-group', 'table-column',
'table-column-group', and 'table-caption'. T and T's siblings may
also be anonymous 'table-row' boxes generated by rule 1.
3. If the parent P of a 'table-column' box T is not a 'table',
'inline-table', or 'table-column-group' box, a box corresponding
to a 'table' element will be generated between P and T. If P is an
'inline' box, then the generated box must be an 'inline-table' box
instead of a 'table' box. This box will span all consecutive
sibling boxes of T that require a 'table' parent: 'table-row',
'table-row-group', 'table-header-group', 'table-footer-group',
'table-column', 'table-column-group', and 'table-caption',
including any anonymous 'table-row' boxes generated by rule 1.
4. If the parent P of a 'table-row-group' (or 'table-header-group',
'table-footer-group', or 'table-column-group' or 'table-caption')
box T is not a 'table' or 'inline-table', a box corresponding to a
'table' element will be generated between P and T. If P is an
'inline' box, then the generated box must be an 'inline-table' box
instead of a 'table' box. This box will span all consecutive
sibling boxess of T that require a 'table' parent: 'table-row',
'table-row-group', 'table-header-group', 'table-footer-group',
'table-column', 'table-column-group', and 'table-caption',
including any anonymous 'table-row' boxes generated by rule 1.
5. If a child T of a 'table' box (or 'inline-table') P is not a
'table-row-group', 'table-header-group', 'table-footer-group',
'table-caption', 'table-column', 'table-column-group' or
'table-row' box, a box corresponding to a 'table-row' element will
be generated between P and T. This box spans all consecutive
siblings of T that are not 'table-row-group',
'table-header-group', 'table-footer-group', 'table-caption',
'table-column', 'table-column-group' or 'table-row' boxes.
6. If a child T of a 'table-row-group' box (or 'table-header-group'
or 'table-footer-group') P is not a 'table-row' box, a box
corresponding to a 'table-row' element will be generated between P
and T. This box spans all consecutive siblings of T that are not
'table-row' boxes.
7. If a child T of a 'table-row' box P is not a 'table-cell' box, a
box corresponding to a 'table-cell' element will be generated
between P and T. This box spans all consecutive siblings of T that
are not 'table-cell' boxes.
Example(s):
In this XML example, a 'table' element is assumed to contain the HBOX
element:
George42871998
because the associated style sheet is:
HBOX { display: table-row }
VBOX { display: table-cell }
Example(s):
In this example, three 'table-cell' elements are assumed to contain
the text in the ROWs. Note that the text is further encapsulated in
anonymous inline boxes, as explained in visual formatting model:
This is the top row.This is the middle row.This is the bottom row.
The style sheet is:
STACK { display: inline-table }
ROW { display: table-row }
D { display: inline; font-weight: bolder }
17.3 Columns
Table cells may belong to two contexts: rows and columns. However, in
the source document cells are descendants of rows, never of columns.
Nevertheless, some aspects of cells can be influenced by setting
properties on columns.
The following properties apply to column and column-group elements:
'border'
The various border properties apply to columns only if
'border-collapse' is set to 'collapse' on the table element. In
that case, borders set on columns and column groups are input
to the conflict resolution algorithm that selects the border
styles at every cell edge.
'background'
The background properties set the background for cells in the
column, but only if both the cell and row have transparent
backgrounds. See "Table layers and transparency."
'width'
The 'width' property gives the minimum width for the column.
'visibility'
If the 'visibility' of a column is set to 'collapse', none of
the cells in the column are rendered, and cells that span into
other columns are clipped. In addition, the width of the table
is diminished by the width the column would have taken up. See
"Dynamic effects" below. Other values for 'visibility' have no
effect.
Example(s):
Here are some examples of style rules that set properties on columns.
The first two rules together implement the "rules" attribute of HTML 4
with a value of "cols". The third rule makes the "totals" column blue,
the final two rules shows how to make a column a fixed size, by using
the fixed layout algorithm.
col { border-style: none solid }
table { border-style: hidden }
col.totals { background: blue }
table { table-layout: fixed }
col.totals { width: 5em }
17.4 Tables in the visual formatting model
In terms of the visual formatting model, a table can behave like a
block-level (for 'display: table') or inline-level (for 'display:
inline-table') element.
In both cases, the table box generates an anonymous box that contains
the table box itself and any caption boxes (in document order). The
caption boxes are block-level boxes that retain their own content,
padding, margin, and border areas, and are rendered as normal blocks
inside the anonymous box. Whether the caption boxes are placed before
or after the table box is decided by the 'caption-side' property, as
described below.
The anonymous box is a 'block' box if the table is block-level, and an
'inline-block' box if the table is inline-level, except that this
block is never considered as a block for 'run-in' interaction, and
that the table box (not the anonymous box) is used when doing baseline
vertical alignment for an 'inline-table'. The width of the anonymous
box is the border-edge width of the table box inside it, as described
by section 17.5.2. Percentages on 'width' and 'height' on the table
are relative to the anonymous box's containing block, not the
anonymous box itself.
The computed values of properties 'position', 'float', 'margin-*',
'top', 'right', 'bottom', and 'left' on the table box are used on the
anonymous box instead of the table box. The table box uses the initial
values for those properties.
A table with a caption above it [D]
Diagram of a table with a caption above it; the top margin of the
caption is collapsed with the top margin of the table.
17.4.1 Caption position and alignment
'caption-side'
Value: top | bottom | inherit
Initial: top
Applies to: 'table-caption' elements
Inherited: yes
Percentages: N/A
Media: visual
Computed value: as specified
This property specifies the position of the caption box with respect
to the table box. Values have the following meanings:
top
Positions the caption box above the table box.
bottom
Positions the caption box below the table box.
Note: CSS2 described a different width and horizontal alignment
behavior. That behavior will be introduced in CSS3 using the values
'top-outside' and 'bottom-outside' on this property.
To align caption content horizontally within the caption box, use the
'text-align' property.
Example(s):
In this example, the 'caption-side' property places captions below
tables. The caption will be as wide as the parent of the table, and
caption text will be left-justified.
caption { caption-side: bottom;
width: auto;
text-align: left }
17.5 Visual layout of table contents
Internal table elements generate rectangular boxes with content and
borders. Cells have padding as well. Internal table elements do not
have margins.
The visual layout of these boxes is governed by a rectangular,
irregular grid of rows and columns. Each box occupies a whole number
of grid cells, determined according to the following rules. These
rules do not apply to HTML 4 or earlier HTML versions; HTML imposes
its own limitations on row and column spans.
1. Each row box occupies one row of grid cells. Together, the row
boxes fill the table from top to bottom in the order they occur in
the source document (i.e., the table occupies exactly as many grid
rows as there are row elements).
2. A row group occupies the same grid cells as the rows it contains.
3. A column box occupies one or more columns of grid cells. Column
boxes are placed next to each other in the order they occur. The
first column box may be either on the left or on the right,
depending on the value of the 'direction' property of the table.
4. A column group box occupies the same grid cells as the columns it
contains.
5. Cells may span several rows or columns. (Although CSS 2.1 doesn't
define how the number of spanned rows or columns is determined, a
user agent may have special knowledge about the source document; a
future update of CSS may provide a way to express this knowledge
in CSS syntax.) Each cell is thus a rectangular box, one or more
grid cells wide and high. The top row of this rectangle is in the
row specified by the cell's parent. The rectangle must be as far
to the left as possible, but the part of the cell in the first
column it occupies must not overlap with any other cell box (i.e.,
a row-spanning cell starting in a prior row), and the cell must be
to the right of all cells in the same row that are earlier in the
source document. If this position would cause a column-spanning
cell to overlap a row-spanning cell from a prior row, CSS does not
define the results: implementations may either overlap the cells
(as is done in many HTML implementations) or may shift the later
cell to the right to avoid such overlap. (This constraint holds if
the 'direction' property of the table is 'ltr'; if the 'direction'
is 'rtl', interchange "left" and "right" in the previous two
sentences.)
6. A cell box cannot extend beyond the last row box of a table or
row-group; the user agents must shorten it until it fits.
The edges of the rows, columns, row groups and column groups in the
collapsing borders model coincide with the hypothetical grid lines on
which the borders of the cells are centered. (And thus, in this model,
the rows together exactly cover the table, leaving no gaps; ditto for
the columns.) In the separated borders model, the edges coincide with
the border edges of cells. (And thus, in this model, there may be gaps
between the rows, columns, row groups or column groups, corresponding
to the 'border-spacing' property.)
Note. Positioning and floating of table cells can cause them not to be
table cells anymore, according to the rules in section 9.7. When
floating is used, the rules on anonymous table objects may cause an
anonymous cell object to be created as well.
Here is an example illustrating rule 5. The following illegal (X)HTML
snippet defines conflicting cells:

1

2

3

4

5

User agents are free to visually overlap the cells, as in the figure
on the left, or to shift the cell to avoid the visual overlap, as in
the figure on the right.
One table with overlapping cells and one without [D]
Two possible renderings of an erroneous HTML table.
17.5.1 Table layers and transparency
For the purposes of finding the background of each table cell, the
different table elements may be thought of as being on six
superimposed layers. The background set on an element in one of the
layers will only be visible if the layers above it have a transparent
background.
schema of table layers [D]
Schema of table layers.
1. The lowest layer is a single plane, representing the table box
itself. Like all boxes, it may be transparent.
2. The next layer contains the column groups. Each column group
extends from the top of the cells in the top row to the bottom of
the cells on the bottom row and from the left edge of its leftmost
column to the right edge of its rightmost column. The background
covers exactly the full area of all cells that originate in the
column group, even if they span outside the column group, but this
difference in area does not affect background image positioning.
3. On top of the column groups are the areas representing the column
boxes. Each column is as tall as the column groups and as wide as
a normal (single-column-spanning) cell in the column. The
background covers exactly the full area of all cells that
originate in the column, even if they span outside the column, but
this difference in area does not affect background image
positioning.
4. Next is the layer containing the row groups. Each row group
extends from the top left corner of its topmost cell in the first
column to the bottom right corner of its bottommost cell in the
last column.
5. The next to last layer contains the rows. Each row is as wide as
the row groups and as tall as a normal (single-row-spanning) cell
in the row. As with columns, the background covers exactly the
full area of all cells that originate in the row, even if they
span outside the row, but this difference in area does not affect
background image positioning.
6. The topmost layer contains the cells themselves. As the figure
shows, although all rows contain the same number of cells, not
every cell may have specified content. In the separated borders
model ('border-collapse' is 'separate'), if the value of their
'empty-cells' property is 'hide' these "empty" cells are
transparent through the cell, row, row group, column and column
group backgrounds, letting the table background show through.
A "missing cell" is a cell in the row/column grid that is not occupied
by an element or pseudo-element. Missing cells are rendered as if an
anonymous table-cell box occupied their position in the grid.
In the following example, the first row contains four non-empty cells,
but the second row contains only one non-empty cell, and thus the
table background shines through, except where a cell from the first
row spans into this row. The following HTML code and style rules
Table example

1

2

3

4

5

might be formatted as follows:
Table with three empty cells in bottom row [D]
Table with empty cells in the bottom row.
Note that if the table has 'border-collapse: separate', the background
of the area given by the 'border-spacing' property is always the
background of the table element. See the separated borders model.
17.5.2 Table width algorithms: the 'table-layout' property
CSS does not define an "optimal" layout for tables since, in many
cases, what is optimal is a matter of taste. CSS does define
constraints that user agents must respect when laying out a table.
User agents may use any algorithm they wish to do so, and are free to
prefer rendering speed over precision, except when the "fixed layout
algorithm" is selected.
Note that this section overrides the rules that apply to calculating
widths as described in section 10.3. In particular, if the margins of
a table are set to '0' and the width to 'auto', the table will not
automatically size to fill its containing block. However, once the
calculated value of 'width' for the table is found (using the
algorithms given below or, when appropriate, some other UA dependant
algorithm) then the other parts of section 10.3 do apply. Therefore a
table can be centered using left and right 'auto' margins, for
instance.
Future updates of CSS may introduce ways of making tables
automatically fit their containing blocks.
'table-layout'
Value: auto | fixed | inherit
Initial: auto
Applies to: 'table' and 'inline-table' elements
Inherited: no
Percentages: N/A
Media: visual
Computed value: as specified
The 'table-layout' property controls the algorithm used to lay out the
table cells, rows, and columns. Values have the following meaning:
fixed
Use the fixed table layout algorithm
auto
Use any automatic table layout algorithm
The two algorithms are described below.
17.5.2.1 Fixed table layout
With this (fast) algorithm, the horizontal layout of the table does
not depend on the contents of the cells; it only depends on the
table's width, the width of the columns, and borders or cell spacing.
The table's width may be specified explicitly with the 'width'
property. A value of 'auto' (for both 'display: table' and 'display:
inline-table') means use the automatic table layout algorithm.
However, if the table is a block-level table ('display: table') in
normal flow, a UA may (but does not have to) use the algorithm of
10.3.3 to compute a width and apply fixed table layout even if the
specified width is 'auto'.
Example(s):
If a UA supports fixed table layout when 'width' is 'auto', the
following will create a table that is 4em narrower than its containing
block:
table { table-layout: fixed;
margin-left: 2em;
margin-right: 2em }
In the fixed table layout algorithm, the width of each column is
determined as follows:
1. A column element with a value other than 'auto' for the 'width'
property sets the width for that column.
2. Otherwise, a cell in the first row with a value other than 'auto'
for the 'width' property determines the width for that column. If
the cell spans more than one column, the width is divided over the
columns.
3. Any remaining columns equally divide the remaining horizontal
table space (minus borders or cell spacing).
The width of the table is then the greater of the value of the 'width'
property for the table element and the sum of the column widths (plus
cell spacing or borders). If the table is wider than the columns, the
extra space should be distributed over the columns.
If a subsequent row has more columns than the greater of the number
determined by the table-column elements and the number determined by
the first row, then additional columns must not be rendered. When
using 'table-layout: fixed', authors should not omit columns from the
first row.
In this manner, the user agent can begin to lay out the table once the
entire first row has been received. Cells in subsequent rows do not
affect column widths. Any cell that has content that overflows uses
the 'overflow' property to determine whether to clip the overflow
content.
17.5.2.2 Automatic table layout
In this algorithm (which generally requires no more than two passes),
the table's width is given by the width of its columns (and
intervening borders). This algorithm reflects the behavior of several
popular HTML user agents at the writing of this specification. UAs are
not required to implement this algorithm to determine the table layout
in the case that 'table-layout' is 'auto'; they can use any other
algorithm even if it results in different behavior.
Input to the automatic table layout must only include the width of the
containing block and the content of, and any CSS properties set on,
the table and any of its descendants.
Note. This may be defined in more detail in CSS3.
The remainder of this section is non-normative.
This algorithm may be inefficient since it requires the user agent to
have access to all the content in the table before determining the
final layout and may demand more than one pass.
Column widths are determined as follows:
1. Calculate the minimum content width (MCW) of each cell: the
formatted content may span any number of lines but may not
overflow the cell box. If the specified 'width' (W) of the cell is
greater than MCW, W is the minimum cell width. A value of 'auto'
means that MCW is the minimum cell width.
Also, calculate the "maximum" cell width of each cell: formatting
the content without breaking lines other than where explicit line
breaks occur.
2. For each column, determine a maximum and minimum column width from
the cells that span only that column. The minimum is that required
by the cell with the largest minimum cell width (or the column
'width', whichever is larger). The maximum is that required by the
cell with the largest maximum cell width (or the column 'width',
whichever is larger).
3. For each cell that spans more than one column, increase the
minimum widths of the columns it spans so that together, they are
at least as wide as the cell. Do the same for the maximum widths.
If possible, widen all spanned columns by approximately the same
amount.
4. For each column group element with a 'width' other than 'auto',
increase the minimum widths of the columns it spans, so that
together they are at least as wide as the column group's 'width'.
This gives a maximum and minimum width for each column. Column widths
influence the final table width as follows:
1. If the 'table' or 'inline-table' element's 'width' property has a
computed value (W) other than 'auto', the property's value as used
for layout is the greater of W and the minimum width required by
all the columns plus cell spacing or borders (MIN). If W is
greater than MIN, the extra width should be distributed over the
columns.
2. If the 'table' or 'inline-table' element has 'width: auto', the
table width used for layout is the greater of the table's
containing block width and MIN. However, if the maximum width
required by the columns plus cell spacing or borders (MAX) is less
than that of the containing block, use MAX.
A percentage value for a column width is relative to the table width.
If the table has 'width: auto', a percentage represents a constraint
on the column's width, which a UA should try to satisfy. (Obviously,
this is not always possible: if the column's width is '110%', the
constraint cannot be satisfied.)
Note. In this algorithm, rows (and row groups) and columns (and column
groups) both constrain and are constrained by the dimensions of the
cells they contain. Setting the width of a column may indirectly
influence the height of a row, and vice versa.
17.5.3 Table height algorithms
The height of a table is given by the 'height' property for the
'table' or 'inline-table' element. A value of 'auto' means that the
height is the sum of the row heights plus any cell spacing or borders.
Any other value is treated as a minimum height. CSS 2.1 does not
define how extra space is distributed when the 'height' property
causes the table to be taller than it otherwise would be. Note. Future
updates of CSS may specify this further.
The height of a 'table-row' element's box is calculated once the user
agent has all the cells in the row available: it is the maximum of the
row's specified 'height' and the minimum height (MIN) required by the
cells. A 'height' value of 'auto' for a 'table-row' means the row
height used for layout is MIN. MIN depends on cell box heights and
cell box alignment (much like the calculation of a line box height).
Percentage heights on table cells, table rows, and table row groups
compute to 'auto'.
In CSS 2.1, the height of a cell box is the maximum of the table
cell's 'height' property and the minimum height required by the
content (MIN). A value of 'auto' for 'height' implies that the value
MIN will be used for layout. CSS 2.1 does not define what percentage
values of 'height' refer to when specified for table cells.
CSS 2.1 does not specify how cells that span more than one row affect
row height calculations except that the sum of the row heights
involved must be great enough to encompass the cell spanning the rows.
The 'vertical-align' property of each table cell determines its
alignment within the row. Each cell's content has a baseline, a top, a
middle, and a bottom, as does the row itself. In the context of
tables, values for 'vertical-align' have the following meanings:
baseline
The baseline of the cell is put at the same height as the
baseline of the first of the rows it spans (see below for the
definition of baselines of cells and rows).
top
The top of the cell box is aligned with the top of the first
row it spans.
bottom
The bottom of the cell box is aligned with the bottom of the
last row it spans.
middle
The center of the cell is aligned with the center of the rows
it spans.
sub, super, text-top, text-bottom, ,
These values do not apply to cells; the cell is aligned at the
baseline instead.
The baseline of a cell is the baseline of the first in-flow line box
in the cell, or the first in-flow table-row in the cell, whichever
comes first. If there is no such line box or table-row, the baseline
is the bottom of content edge of the cell box. For the purposes of
finding a baseline, in-flow boxes with a scrolling mechanisms (see the
'overflow' property) must be considered as if scrolled to their origin
position. Note that the baseline of a cell may end up below its bottom
border, see the example below.
The maximum distance between the top of the cell box and the baseline
over all cells that have 'vertical-align: baseline' is used to set the
baseline of the row. Here is an example:
Example of vertically aligning the cells [D]
Diagram showing the effect of various values of 'vertical-align' on
table cells.
Cell boxes 1 and 2 are aligned at their baselines. Cell box 2 has the
largest height above the baseline, so that determines the baseline of
the row.
If a row has no cell box aligned to its baseline, the baseline of that
row is the bottom content edge of the lowest cell in the row.
To avoid ambiguous situations, the alignment of cells proceeds in the
following order:
1. First the cells that are aligned on their baseline are positioned.
This will establish the baseline of the row. Next the cells with
'vertical-align: top' are positioned.
2. The row now has a top, possibly a baseline, and a provisional
height, which is the distance from the top to the lowest bottom of
the cells positioned so far. (See conditions on the cell padding
below.)
3. If any of the remaining cells, those aligned at the bottom or the
middle, have a height that is larger than the current height of
the row, the height of the row will be increased to the maximum of
those cells, by lowering the bottom.
4. Finally the remaining cells are positioned.
Cell boxes that are smaller than the height of the row receive extra
top or bottom padding.
The cell in this example has a baseline below its bottom border:
div { height: 0; overflow: hidden; }

Test

17.5.4 Horizontal alignment in a column
The horizontal alignment of a cell's content within a cell box is
specified with the 'text-align' property.
17.5.5 Dynamic row and column effects
The 'visibility' property takes the value 'collapse' for row, row
group, column, and column group elements. This value causes the entire
row or column to be removed from the display, and the space normally
taken up by the row or column to be made available for other content.
Contents of spanned rows and columns that intersect the collapsed
column or row are clipped. The suppression of the row or column,
however, does not otherwise affect the layout of the table. This
allows dynamic effects to remove table rows or columns without forcing
a re-layout of the table in order to account for the potential change
in column constraints.
17.6 Borders
There are two distinct models for setting borders on table cells in
CSS. One is most suitable for so-called separated borders around
individual cells, the other is suitable for borders that are
continuous from one end of the table to the other. Many border styles
can be achieved with either model, so it is often a matter of taste
which one is used.
'border-collapse'
Value: collapse | separate | inherit
Initial: separate
Applies to: 'table' and 'inline-table' elements
Inherited: yes
Percentages: N/A
Media: visual
Computed value: as specified
This property selects a table's border model. The value 'separate'
selects the separated borders border model. The value 'collapse'
selects the collapsing borders model. The models are described below.
17.6.1 The separated borders model
'border-spacing'
Value: ? | inherit
Initial: 0
Applies to: 'table' and 'inline-table' elements*
Inherited: yes
Percentages: N/A
Media: visual
Computed value: two absolute lengths
*) Note: user agents may also apply the 'border-spacing' property to
'frameset' elements. Which elements are 'frameset' elements is not
defined by this specification and is up to the document language. For
example, HTML4 defines a element, and XHTML 1.0 defines a
element. The 'border-spacing' property on a 'frameset'
element can be thus used as a valid substitute for the non-standard
'framespacing' attribute.
The lengths specify the distance that separates adjoining cell
borders. If one length is specified, it gives both the horizontal and
vertical spacing. If two are specified, the first gives the horizontal
spacing and the second the vertical spacing. Lengths may not be
negative.
The distance between the table border and the borders of the cells on
the edge of the table is the table's padding for that side, plus the
relevant border spacing distance. For example, on the right hand side,
the distance is padding-right + horizontal border-spacing.
The width of the table is the distance from the left inner padding
edge to the right inner padding edge (including the border spacing but
excluding padding and border).
However, in HTML and XHTML1, the width of the

element is the
distance from the left border edge to the right border edge.
Note: In CSS3 this peculiar requirement will be defined in terms of UA
style sheet rules and the 'box-sizing' property.
In this model, each cell has an individual border. The
'border-spacing' property specifies the distance between the borders
of adjoining cells. In this space, the row, column, row group, and
column group backgrounds are invisible, allowing the table background
to show through. Rows, columns, row groups, and column groups cannot
have borders (i.e., user agents must ignore the border properties for
those elements).
Example(s):
The table in the figure below could be the result of a style sheet
like this:
table { border: outset 10pt;
border-collapse: separate;
border-spacing: 15pt }
td { border: inset 5pt }
td.special { border: inset 10pt } /* The top-left cell */
A table with border-spacing [D]
A table with 'border-spacing' set to a length value. Note that each
cell has its own border, and the table has a separate border as well.
17.6.1.1 Borders and Backgrounds around empty cells: the 'empty-cells'
property
'empty-cells'
Value: show | hide | inherit
Initial: show
Applies to: 'table-cell' elements
Inherited: yes
Percentages: N/A
Media: visual
Computed value: as specified
In the separated borders model, this property controls the rendering
of borders and backgrounds around cells that have no visible content.
Empty cells and cells with the 'visibility' property set to 'hidden'
are considered to have no visible content. Cells are empty unless they
contain one or more of the following:
* floating content (including empty elements),
* in-flow content (including empty elements) other than whitespace
that has been collapsed away by the 'white-space' property
handling.
When this property has the value 'show', borders and backgrounds are
drawn around/behind empty cells (like normal cells).
A value of 'hide' means that no borders or backgrounds are drawn
around/behind empty cells (see point 6 in 17.5.1). Furthermore, if all
the cells in a row have a value of 'hide' and have no visible content,
then the row has zero height and there is vertical border-spacing on
only one side of the row.
Example(s):
The following rule causes borders and backgrounds to be drawn around
all cells:
table { empty-cells: show }
17.6.2 The collapsing border model
In the collapsing border model, it is possible to specify borders that
surround all or part of a cell, row, row group, column, and column
group. Borders for HTML's "rule" attribute can be specified this way.
Borders are centered on the grid lines between the cells. User agents
must find a consistent rule for rounding off in the case of an odd
number of discrete units (screen pixels, printer dots).
The diagram below shows how the width of the table, the widths of the
borders, the padding, and the cell width interact. Their relation is
given by the following equation, which holds for every row of the
table:
row-width = (0.5 * border-width[0]) + padding-left[1] + width[1] +
padding-right[1] + border-width[1] + padding-left[2] +...+
padding-right[n] + (0.5 * border-width[n])
Here n is the number of cells in the row, padding-left[i] and
padding-right[i] refer to the left (resp., right) padding of cell i,
and border-width[i] refers to the border between cells i and i + 1.
UAs must compute an initial left and right border width for the table
by examining the first and last cells in the first row of the table.
The left border width of the table is half of the first cell's
collapsed left border, and the right border width of the table is half
of the last cell's collapsed right border. If subsequent rows have
larger collapsed left and right borders, then any excess spills into
the margin area of the table.
The top border width of the table is computed by examining all cells
who collapse their top borders with the top border of the table. The
top border width of the table is equal to half of the maximum
collapsed top border. The bottom border width is computed by examining
all cells whose bottom borders collapse with the bottom of the table.
The bottom border width is equal to half of the maximum collapsed
bottom border.
Any borders that spill into the margin are taken into account when
determining if the table overflows some ancestor (see 'overflow').
Schema showing the widths of cells and borders and the padding of
cells [D]
Schema showing the widths of cells and borders and the padding of
cells.
Note that in this model, the width of the table includes half the
table border. Also, in this model, a table doesn't have padding (but
does have margins).
CSS 2.1 does not define where the edge of a background on a table
element lies.
17.6.2.1 Border conflict resolution
In the collapsing border model, borders at every edge of every cell
may be specified by border properties on a variety of elements that
meet at that edge (cells, rows, row groups, columns, column groups,
and the table itself), and these borders may vary in width, style, and
color. The rule of thumb is that at each edge the most "eye catching"
border style is chosen, except that any occurrence of the style
'hidden' unconditionally turns the border off.
The following rules determine which border style "wins" in case of a
conflict:
1. Borders with the 'border-style' of 'hidden' take precedence over
all other conflicting borders. Any border with this value
suppresses all borders at this location.
2. Borders with a style of 'none' have the lowest priority. Only if
the border properties of all the elements meeting at this edge are
'none' will the border be omitted (but note that 'none' is the
default value for the border style.)
3. If none of the styles are 'hidden' and at least one of them is not
'none', then narrow borders are discarded in favor of wider ones.
If several have the same 'border-width' then styles are preferred
in this order: 'double', 'solid', 'dashed', 'dotted', 'ridge',
'outset', 'groove', and the lowest: 'inset'.
4. If border styles differ only in color, then a style set on a cell
wins over one on a row, which wins over a row group, column,
column group and, lastly, table. When two elements of the same
type conflict, then the one further to the left (if the table's
'direction' is 'ltr'; right, if it is 'rtl') and further to the
top wins.
Example(s):
The following example illustrates the application of these precedence
rules. This style sheet:
table { border-collapse: collapse;
border: 5px solid yellow; }
*#col1 { border: 3px solid black; }
td { border: 1px solid red; padding: 1em; }
td.cell5 { border: 5px dashed blue; }
td.cell6 { border: 5px solid green; }
with this HTML source:

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

would produce something like this:
An example of a table with collapsed borders [D]
An example of a table with collapsed borders.
Example(s):
Here is an example of hidden collapsing borders:
Table with two omitted borders [D]
Table with two omitted internal borders.
HTML source:

foo

bar

foo

bar

17.6.3 Border styles
Some of the values of the 'border-style' have different meanings in
tables than for other elements. In the list below they are marked with
an asterisk.
none
No border.
*hidden
Same as 'none', but in the collapsing border model, also
inhibits any other border (see the section on border
conflicts).
dotted
The border is a series of dots.
dashed
The border is a series of short line segments.
solid
The border is a single line segment.
double
The border is two solid lines. The sum of the two lines and the
space between them equals the value of 'border-width'.
groove
The border looks as though it were carved into the canvas.
ridge
The opposite of 'groove': the border looks as though it were
coming out of the canvas.
*inset
In the separated borders model, the border makes the entire box
look as though it were embedded in the canvas. In the
collapsing border model, same as 'ridge'.
*outset
In the separated borders model, the border makes the entire box
look as though it were coming out of the canvas. In the
collapsing border model, same as 'groove'.
_________________________________________________________________
_________________________________________________________________
18 User interface
Contents
* 18.1 Cursors: the 'cursor' property
* 18.2 System Colors
* 18.3 User preferences for fonts
* 18.4 Dynamic outlines: the 'outline' property
+ 18.4.1 Outlines and the focus
* 18.5 Magnification
18.1 Cursors: the 'cursor' property
'cursor'
Value: [ [ ,]* [ auto | crosshair | default | pointer | move |
e-resize | ne-resize | nw-resize | n-resize | se-resize | sw-resize |
s-resize | w-resize | text | wait | help | progress ] ] | inherit
Initial: auto
Applies to: all elements
Inherited: yes
Percentages: N/A
Media: visual, interactive
Computed value: as specified, except with any relative URLs
converted to absolute
This property specifies the type of cursor to be displayed for the
pointing device. Values have the following meanings:
auto
The UA determines the cursor to display based on the current
context.
crosshair
A simple crosshair (e.g., short line segments resembling a "+"
sign).
default
The platform-dependent default cursor. Often rendered as an
arrow.
pointer
The cursor is a pointer that indicates a link.
move
Indicates something is to be moved.
e-resize, ne-resize, nw-resize, n-resize, se-resize, sw-resize,
s-resize, w-resize
Indicate that some edge is to be moved. For example, the
'se-resize' cursor is used when the movement starts from the
south-east corner of the box.
text
Indicates text that may be selected. Often rendered as an
I-beam.
wait
Indicates that the program is busy and the user should wait.
Often rendered as a watch or hourglass.
progress
A progress indicator. The program is performing some
processing, but is different from 'wait' in that the user may
still interact with the program. Often rendered as a spinning
beach ball, or an arrow with a watch or hourglass.
help
Help is available for the object under the cursor. Often
rendered as a question mark or a balloon.
The user agent retrieves the cursor from the resource
designated by the URI. If the user agent cannot handle the
first cursor of a list of cursors, it should attempt to handle
the second, etc. If the user agent cannot handle any
user-defined cursor, it must use the generic cursor at the end
of the list.
Example(s):
:link,:visited { cursor: url(example.svg#linkcursor), url(hyper.cur), pointer }
This example sets the cursor on all hyperlinks (whether visited or
not) to an external SVG cursor. User agents that don't support SVG
cursors would simply skip to the next value and attempt to use the
"hyper.cur" cursor. If that cursor format was also not supported, the
UA would skip to the next value and simply render the 'pointer'
cursor.
18.2 System Colors
Note. The System Colors are deprecated in the CSS3 Color Module
[CSS3COLOR].
In addition to being able to assign pre-defined color values to text,
backgrounds, etc., CSS2 introduced a set of named color values that
allows authors to specify colors in a manner that integrates them into
the operating system's graphic environment.
For systems that do not have a corresponding value, the specified
value should be mapped to the nearest system value, or to a default
color.
The following lists additional values for color-related CSS properties
and their general meaning. Any color property (e.g., 'color' or
'background-color') can take one of the following names. Although
these are case-insensitive, it is recommended that the mixed
capitalization shown below be used, to make the names more legible.
ActiveBorder
Active window border.
ActiveCaption
Active window caption.
AppWorkspace
Background color of multiple document interface.
Background
Desktop background.
ButtonFace
Face color for three-dimensional display elements.
ButtonHighlight
Highlight color for three-dimensional display elements (for
edges facing away from the light source).
ButtonShadow
Shadow color for three-dimensional display elements.
ButtonText
Text on push buttons.
CaptionText
Text in caption, size box, and scrollbar arrow box.
GrayText
Grayed (disabled) text. This color is set to #000 if the
current display driver does not support a solid gray color.
Highlight
Item(s) selected in a control.
HighlightText
Text of item(s) selected in a control.
InactiveBorder
Inactive window border.
InactiveCaption
Inactive window caption.
InactiveCaptionText
Color of text in an inactive caption.
InfoBackground
Background color for tooltip controls.
InfoText
Text color for tooltip controls.
Menu
Menu background.
MenuText
Text in menus.
Scrollbar
Scroll bar gray area.
ThreeDDarkShadow
Dark shadow for three-dimensional display elements.
ThreeDFace
Face color for three-dimensional display elements.
ThreeDHighlight
Highlight color for three-dimensional display elements.
ThreeDLightShadow
Light color for three-dimensional display elements (for edges
facing the light source).
ThreeDShadow
Dark shadow for three-dimensional display elements.
Window
Window background.
WindowFrame
Window frame.
WindowText
Text in windows.
Example(s):
For example, to set the foreground and background colors of a
paragraph to the same foreground and background colors of the user's
window, write the following:
p { color: WindowText; background-color: Window }
18.3 User preferences for fonts
As for colors, authors may specify fonts in a way that makes use of a
user's system resources. Please consult the 'font' property for
details.
18.4 Dynamic outlines: the 'outline' property
At times, style sheet authors may want to create outlines around
visual objects such as buttons, active form fields, image maps, etc.,
to make them stand out. CSS 2.1 outlines differ from borders in the
following ways:
1. Outlines do not take up space.
2. Outlines may be non-rectangular.
The outline properties control the style of these dynamic outlines.
'outline'
Value: [ || ||
] | inherit
Initial: see individual properties
Applies to: all elements
Inherited: no
Percentages: N/A
Media: visual, interactive
Computed value: see individual properties
'outline-width'
Value: | inherit
Initial: medium
Applies to: all elements
Inherited: no
Percentages: N/A
Media: visual, interactive
Computed value: absolute length; '0' if the outline style is 'none'
'outline-style'
Value: | inherit
Initial: none
Applies to: all elements
Inherited: no
Percentages: N/A
Media: visual, interactive
Computed value: as specified
'outline-color'
Value: | invert | inherit
Initial: invert
Applies to: all elements
Inherited: no
Percentages: N/A
Media: visual, interactive
Computed value: as specified
The outline created with the outline properties is drawn "over" a box,
i.e., the outline is always on top, and doesn't influence the position
or size of the box, or of any other boxes. Therefore, displaying or
suppressing outlines does not cause reflow or overflow.
The outline may be drawn starting just outside the border edge.
Outlines may be non-rectangular. For example, if the element is broken
across several lines, the outline is the minimum outline that encloses
all the element's boxes. In contrast to borders, the outline is not
open at the line box's end or start, but is always fully connected if
possible.
The 'outline-width' property accepts the same values as
'border-width'.
The 'outline-style' property accepts the same values as
'border-style', except that 'hidden' is not a legal outline style.
The 'outline-color' accepts all colors, as well as the keyword
'invert'. 'Invert' is expected to perform a color inversion on the
pixels on the screen. This is a common trick to ensure the focus
border is visible, regardless of color background.
Conformant UAs may ignore the 'invert' value on platforms that do not
support color inversion of the pixels on the screen. If the UA does
not support the 'invert' value then the initial value of the
'outline-color' property is the value of the 'color' property, similar
to the initial value of the 'border-top-color' property.
The 'outline' property is a shorthand property, and sets all three of
'outline-style', 'outline-width', and 'outline-color'.
Note. The outline is the same on all sides. In contrast to borders,
there is no 'outline-top' or 'outline-left' property.
This specification does not define how multiple overlapping outlines
are drawn, or how outlines are drawn for boxes that are partially
obscured behind other elements.
Note. Since the outline does not affect formatting (i.e., no space is
left for it in the box model), it may well overlap other elements on
the page.
Example(s):
Here's an example of drawing a thick outline around a BUTTON element:
button { outline : thick solid}
Scripts may be used to dynamically change the width of the outline,
without provoking a reflow.
18.4.1 Outlines and the focus
Graphical user interfaces may use outlines around elements to tell the
user which element on the page has the focus. These outlines are in
addition to any borders, and switching outlines on and off should not
cause the document to reflow. The focus is the subject of user
interaction in a document (e.g., for entering text, selecting a
button, etc.). User agents supporting the interactive media group must
keep track of where the focus lies and must also represent the focus.
This may be done by using dynamic outlines in conjunction with the
:focus pseudo-class.
Example(s):
For example, to draw a thick black line around an element when it has
the focus, and a thick red line when it is active, the following rules
can be used:
:focus { outline: thick solid black }
:active { outline: thick solid red }
18.5 Magnification
The CSS working group considers that the magnification of a document
or portions of a document should not be specified through style
sheets. User agents may support such magnification in different ways
(e.g., larger images, louder sounds, etc.)
When magnifying a page, UAs should preserve the relationships between
positioned elements. For example, a comic strip may be composed of
images with overlaid text elements. When magnifying this page, a user
agent should keep the text within the comic strip balloon.
_________________________________________________________________
_________________________________________________________________
Appendix A. Aural style sheets
Contents
* A.1 The media types 'aural' and 'speech'
* A.2 Introduction to aural style sheets
+ A.2.1 Angles
+ A.2.2 Times
+ A.2.3 Frequencies
* A.3 Volume properties: 'volume'
* A.4 Speaking properties: 'speak'
* A.5 Pause properties: 'pause-before', 'pause-after', and 'pause'
* A.6 Cue properties: 'cue-before', 'cue-after', and 'cue'
* A.7 Mixing properties: 'play-during'
* A.8 Spatial properties: 'azimuth' and 'elevation'
* A.9 Voice characteristic properties: 'speech-rate',
'voice-family', 'pitch', 'pitch-range', 'stress', and 'richness'
* A.10 Speech properties: 'speak-punctuation' and 'speak-numeral'
* A.11 Audio rendering of tables
+ A.11.1 Speaking headers: the 'speak-header' property
* A.12 Sample style sheet for HTML
* A.13 Emacspeak
This chapter is informative. UAs are not required to implement the
properties of this chapter in order to conform to CSS 2.1.
A.1 The media types 'aural' and 'speech'
We expect that in a future level of CSS there will be new properties
and values defined for speech output. Therefore CSS 2.1 reserves the
'speech' media type (see chapter 7, "Media types"), but does not yet
define which properties do or do not apply to it.
The properties in this appendix apply to a media type 'aural', that
was introduced in CSS2. The type 'aural' is now deprecated.
This means that a style sheet such as
@media speech {
body { voice-family: Paul }
}
is valid, but that its meaning is not defined by CSS 2.1, while
@media aural {
body { voice-family: Paul }
}
is deprecated, but defined by this appendix.
A.2 Introduction to aural style sheets
The aural rendering of a document, already commonly used by the blind
and print-impaired communities, combines speech synthesis and
"auditory icons." Often such aural presentation occurs by converting
the document to plain text and feeding this to a screen reader --
software or hardware that simply reads all the characters on the
screen. This results in less effective presentation than would be the
case if the document structure were retained. Style sheet properties
for aural presentation may be used together with visual properties
(mixed media) or as an aural alternative to visual presentation.
Besides the obvious accessibility advantages, there are other large
markets for listening to information, including in-car use, industrial
and medical documentation systems (intranets), home entertainment, and
to help users learning to read or who have difficulty reading.
When using aural properties, the canvas consists of a
three-dimensional physical space (sound surrounds) and a temporal
space (one may specify sounds before, during, and after other sounds).
The CSS properties also allow authors to vary the quality of
synthesized speech (voice type, frequency, inflection, etc.).
Example(s):
h1, h2, h3, h4, h5, h6 {
voice-family: paul;
stress: 20;
richness: 90;
cue-before: url("ping.au")
}
p.heidi { azimuth: center-left }
p.peter { azimuth: right }
p.goat { volume: x-soft }
This will direct the speech synthesizer to speak headers in a voice (a
kind of "audio font") called "paul", on a flat tone, but in a very
rich voice. Before speaking the headers, a sound sample will be played
from the given URL. Paragraphs with class "heidi" will appear to come
from front left (if the sound system is capable of spatial audio), and
paragraphs of class "peter" from the right. Paragraphs with class
"goat" will be very soft.
A.2.1 Angles
Angle values are denoted by in the text. Their format is a
immediately followed by an angle unit identifier.
Angle unit identifiers are:
* deg: degrees
* grad: grads
* rad: radians
Angle values may be negative. They should be normalized to the range
0-360deg by the user agent. For example, -10deg and 350deg are
equivalent.
For example, a right angle is '90deg' or '100grad' or
'1.570796326794897rad'.
Like for , the unit may be omitted, if the value is zero:
'0deg' may be written as '0'.
A.2.2 Times
Time values are denoted by in the text. Their format is a
immediately followed by a time unit identifier.
Time unit identifiers are:
* ms: milliseconds
* s: seconds
Time values may not be negative.
Like for , the unit may be omitted, if the value is zero: '0s'
may be written as '0'.
A.2.3 Frequencies
Frequency values are denoted by in the text. Their format
is a immediately followed by a frequency unit identifier.
Frequency unit identifiers are:
* Hz: Hertz
* kHz: kilohertz
Frequency values may not be negative.
For example, 200Hz (or 200hz) is a bass sound, and 6kHz is a treble
sound.
Like for , the unit may be omitted, if the value is zero:
'0Hz' may be written as '0'.
A.3 Volume properties: 'volume'
'volume'
Value: | | silent | x-soft | soft | medium |
loud | x-loud | inherit
Initial: medium
Applies to: all elements
Inherited: yes
Percentages: refer to inherited value
Media: aural
Computed value: number
Volume refers to the median volume of the waveform. In other words, a
highly inflected voice at a volume of 50 might peak well above that.
The overall values are likely to be human adjustable for comfort, for
example with a physical volume control (which would increase both the
0 and 100 values proportionately); what this property does is adjust
the dynamic range.
Values have the following meanings:
Any number between '0' and '100'. '0' represents the minimum
audible volume level and 100 corresponds to the maximum
comfortable level.
Percentage values are calculated relative to the inherited
value, and are then clipped to the range '0' to '100'.
silent
No sound at all. The value '0' does not mean the same as
'silent'.
x-soft
Same as '0'.
soft
Same as '25'.
medium
Same as '50'.
loud
Same as '75'.
x-loud
Same as '100'.
User agents should allow the values corresponding to '0' and '100' to
be set by the listener. No one setting is universally applicable;
suitable values depend on the equipment in use (speakers, headphones),
the environment (in car, home theater, library) and personal
preferences. Some examples:
* A browser for in-car use has a setting for when there is lots of
background noise. '0' would map to a fairly high level and '100'
to a quite high level. The speech is easily audible over the road
noise but the overall dynamic range is compressed. Cars with
better insulation might allow a wider dynamic range.
* Another speech browser is being used in an apartment, late at
night, or in a shared study room. '0' is set to a very quiet level
and '100' to a fairly quiet level, too. As with the first example,
there is a low slope; the dynamic range is reduced. The actual
volumes are low here, whereas they were high in the first example.
* In a quiet and isolated house, an expensive hi-fi home theater
setup. '0' is set fairly low and '100' to quite high; there is
wide dynamic range.
The same author style sheet could be used in all cases, simply by
mapping the '0' and '100' points suitably at the client side.
A.4 Speaking properties: 'speak'
'speak'
Value: normal | none | spell-out | inherit
Initial: normal
Applies to: all elements
Inherited: yes
Percentages: N/A
Media: aural
Computed value: as specified
This property specifies whether text will be rendered aurally and if
so, in what manner. The possible values are:
none
Suppresses aural rendering so that the element requires no time
to render. Note, however, that descendants may override this
value and will be spoken. (To be sure to suppress rendering of
an element and its descendants, use the 'display' property).
normal
Uses language-dependent pronunciation rules for rendering an
element and its children.
spell-out
Spells the text one letter at a time (useful for acronyms and
abbreviations).
Note the difference between an element whose 'volume' property has a
value of 'silent' and an element whose 'speak' property has the value
'none'. The former takes up the same time as if it had been spoken,
including any pause before and after the element, but no sound is
generated. The latter requires no time and is not rendered (though its
descendants may be).
A.5 Pause properties: 'pause-before', 'pause-after', and 'pause'
'pause-before'
Value: | | inherit
Initial: 0
Applies to: all elements
Inherited: no
Percentages: see prose
Media: aural
Computed value: time
'pause-after'
Value: | | inherit
Initial: 0
Applies to: all elements
Inherited: no
Percentages: see prose
Media: aural
Computed value: time;;
These properties specify a pause to be observed before (or after)
speaking an element's content. Values have the following meanings:
Note. In CSS3 pauses are inserted around the cues and content rather
than between them. See [CSS3SPEECH] for details.
Expresses the pause in absolute time units (seconds and
milliseconds).
Refers to the inverse of the value of the 'speech-rate'
property. For example, if the speech-rate is 120 words per
minute (i.e., a word takes half a second, or 500ms) then a
'pause-before' of 100% means a pause of 500 ms and a
'pause-before' of 20% means 100ms.
The pause is inserted between the element's content and any
'cue-before' or 'cue-after' content.
Authors should use relative units to create more robust style sheets
in the face of large changes in speech-rate.
'pause'
Value: [ [ | ]{1,2} ] | inherit
Initial: see individual properties
Applies to: all elements
Inherited: no
Percentages: see descriptions of 'pause-before' and 'pause-after'
Media: aural
Computed value: see individual properties
The 'pause' property is a shorthand for setting 'pause-before' and
'pause-after'. If two values are given, the first value is
'pause-before' and the second is 'pause-after'. If only one value is
given, it applies to both properties.
Example(s):
h1 { pause: 20ms } /* pause-before: 20ms; pause-after: 20ms */
h2 { pause: 30ms 40ms } /* pause-before: 30ms; pause-after: 40ms */
h3 { pause-after: 10ms } /* pause-before unspecified; pause-after: 10ms */
A.6 Cue properties: 'cue-before', 'cue-after', and 'cue'
'cue-before'
Value: | none | inherit
Initial: none
Applies to: all elements
Inherited: no
Percentages: N/A
Media: aural
Computed value: absolute URI or 'none'
'cue-after'
Value: | none | inherit
Initial: none
Applies to: all elements
Inherited: no
Percentages: N/A
Media: aural
Computed value: absolute URI or 'none'
Auditory icons are another way to distinguish semantic elements.
Sounds may be played before and/or after the element to delimit it.
Values have the following meanings:
The URI must designate an auditory icon resource. If the URI
resolves to something other than an audio file, such as an
image, the resource should be ignored and the property treated
as if it had the value 'none'.
none
No auditory icon is specified.
Example(s):
a {cue-before: url("bell.aiff"); cue-after: url("dong.wav") }
h1 {cue-before: url("pop.au"); cue-after: url("pop.au") }
'cue'
Value: [ || ] | inherit
Initial: see individual properties
Applies to: all elements
Inherited: no
Percentages: N/A
Media: aural
Computed value: see individual properties
The 'cue' property is a shorthand for setting 'cue-before' and
'cue-after'. If two values are given, the first value is 'cue-before'
and the second is 'cue-after'. If only one value is given, it applies
to both properties.
Example(s):
The following two rules are equivalent:
h1 {cue-before: url("pop.au"); cue-after: url("pop.au") }
h1 {cue: url("pop.au") }
If a user agent cannot render an auditory icon (e.g., the user's
environment does not permit it), we recommend that it produce an
alternative cue.
Please see the sections on the :before and :after pseudo-elements for
information on other content generation techniques. 'cue-before'
sounds and 'pause-before' gaps are inserted before content from the
':before' pseudo-element. Similarly, 'pause-after' gaps and
'cue-after' sounds are inserted after content from the ':after'
pseudo-element.
A.7 Mixing properties: 'play-during'
'play-during'
Value: [ mix || repeat ]? | auto | none | inherit
Initial: auto
Applies to: all elements
Inherited: no
Percentages: N/A
Media: aural
Computed value: absolute URI, rest as specified
Similar to the 'cue-before' and 'cue-after' properties, this property
specifies a sound to be played as a background while an element's
content is spoken. Values have the following meanings:
The sound designated by this is played as a background
while the element's content is spoken.
mix
When present, this keyword means that the sound inherited from
the parent element's 'play-during' property continues to play
and the sound designated by the is mixed with it. If
'mix' is not specified, the element's background sound replaces
the parent's.
repeat
When present, this keyword means that the sound will repeat if
it is too short to fill the entire duration of the element.
Otherwise, the sound plays once and then stops. This is similar
to the 'background-repeat' property. If the sound is too long
for the element, it is clipped once the element has been
spoken.
auto
The sound of the parent element continues to play (it is not
restarted, which would have been the case if this property had
been inherited).
none
This keyword means that there is silence. The sound of the
parent element (if any) is silent during the current element
and continues after the current element.
Example(s):
blockquote.sad { play-during: url("violins.aiff") }
blockquote Q { play-during: url("harp.wav") mix }
span.quiet { play-during: none }
A.8 Spatial properties: 'azimuth' and 'elevation'
Spatial audio is an important stylistic property for aural
presentation. It provides a natural way to tell several voices apart,
as in real life (people rarely all stand in the same spot in a room).
Stereo speakers produce a lateral sound stage. Binaural headphones or
the increasingly popular 5-speaker home theater setups can generate
full surround sound, and multi-speaker setups can create a true
three-dimensional sound stage. VRML 2.0 also includes spatial audio,
which implies that in time consumer-priced spatial audio hardware will
become more widely available.
'azimuth'
Value: | [[ left-side | far-left | left | center-left |
center | center-right | right | far-right | right-side ] || behind ] |
leftwards | rightwards | inherit
Initial: center
Applies to: all elements
Inherited: yes
Percentages: N/A
Media: aural
Computed value: normalized angle
Values have the following meanings:
Position is described in terms of an angle within the range
'-360deg' to '360deg'. The value '0deg' means directly ahead in
the center of the sound stage. '90deg' is to the right,
'180deg' behind, and '270deg' (or, equivalently and more
conveniently, '-90deg') to the left.
left-side
Same as '270deg'. With 'behind', '270deg'.
far-left
Same as '300deg'. With 'behind', '240deg'.
left
Same as '320deg'. With 'behind', '220deg'.
center-left
Same as '340deg'. With 'behind', '200deg'.
center
Same as '0deg'. With 'behind', '180deg'.
center-right
Same as '20deg'. With 'behind', '160deg'.
right
Same as '40deg'. With 'behind', '140deg'.
far-right
Same as '60deg'. With 'behind', '120deg'.
right-side
Same as '90deg'. With 'behind', '90deg'.
leftwards
Moves the sound to the left, relative to the current angle.
More precisely, subtracts 20 degrees. Arithmetic is carried out
modulo 360 degrees. Note that 'leftwards' is more accurately
described as "turned counter-clockwise," since it always
subtracts 20 degrees, even if the inherited azimuth is already
behind the listener (in which case the sound actually appears
to move to the right).
rightwards
Moves the sound to the right, relative to the current angle.
More precisely, adds 20 degrees. See 'leftwards' for
arithmetic.
This property is most likely to be implemented by mixing the same
signal into different channels at differing volumes. It might also use
phase shifting, digital delay, and other such techniques to provide
the illusion of a sound stage. The precise means used to achieve this
effect and the number of speakers used to do so are user
agent-dependent; this property merely identifies the desired end
result.
Example(s):
h1 { azimuth: 30deg }
td.a { azimuth: far-right } /* 60deg */
#12 { azimuth: behind far-right } /* 120deg */
p.comment { azimuth: behind } /* 180deg */
If spatial-azimuth is specified and the output device cannot produce
sounds behind the listening position, user agents should convert
values in the rearwards hemisphere to forwards hemisphere values. One
method is as follows:
* if 90deg < x <= 180deg then x := 180deg - x
* if 180deg < x <= 270deg then x := 540deg - x
'elevation'
Value: | below | level | above | higher | lower | inherit
Initial: level
Applies to: all elements
Inherited: yes
Percentages: N/A
Media: aural
Computed value: normalized angle
Values of this property have the following meanings:
Specifies the elevation as an angle, between '-90deg' and
'90deg'. '0deg' means on the forward horizon, which loosely
means level with the listener. '90deg' means directly overhead
and '-90deg' means directly below.
below
Same as '-90deg'.
level
Same as '0deg'.
above
Same as '90deg'.
higher
Adds 10 degrees to the current elevation.
lower
Subtracts 10 degrees from the current elevation.
The precise means used to achieve this effect and the number of
speakers used to do so are undefined. This property merely identifies
the desired end result.
Example(s):
h1 { elevation: above }
tr.a { elevation: 60deg }
tr.b { elevation: 30deg }
tr.c { elevation: level }
A.9 Voice characteristic properties: 'speech-rate', 'voice-family', 'pitch',
'pitch-range', 'stress', and 'richness'
'speech-rate'
Value: | x-slow | slow | medium | fast | x-fast | faster |
slower | inherit
Initial: medium
Applies to: all elements
Inherited: yes
Percentages: N/A
Media: aural
Computed value: number
This property specifies the speaking rate. Note that both absolute and
relative keyword values are allowed (compare with 'font-size'). Values
have the following meanings:
Specifies the speaking rate in words per minute, a quantity
that varies somewhat by language but is nevertheless widely
supported by speech synthesizers.
x-slow
Same as 80 words per minute.
slow
Same as 120 words per minute
medium
Same as 180 - 200 words per minute.
fast
Same as 300 words per minute.
x-fast
Same as 500 words per minute.
faster
Adds 40 words per minute to the current speech rate.
slower
Subtracts 40 words per minutes from the current speech rate.
'voice-family'
Value: [[ | ],]* [ |
] | inherit
Initial: depends on user agent
Applies to: all elements
Inherited: yes
Percentages: N/A
Media: aural
Computed value: as specified
The value is a comma-separated, prioritized list of voice family names
(compare with 'font-family'). Values have the following meanings:
Values are voice families. Possible values are 'male',
'female', and 'child'.
Values are specific instances (e.g., comedian, trinoids,
carlos, lani).
Example(s):
h1 { voice-family: announcer, male }
p.part.romeo { voice-family: romeo, male }
p.part.juliet { voice-family: juliet, female }
Names of specific voices may be quoted, and indeed must be quoted if
any of the words that make up the name does not conform to the syntax
rules for identifiers. It is also recommended to quote specific voices
with a name consisting of more than one word. If quoting is omitted,
any whitespace characters before and after the voice family name are
ignored and any sequence of whitespace characters inside the voice
family name is converted to a single space.
'pitch'
Value: | x-low | low | medium | high | x-high | inherit
Initial: medium
Applies to: all elements
Inherited: yes
Percentages: N/A
Media: aural
Computed value: frequency
Specifies the average pitch (a frequency) of the speaking voice. The
average pitch of a voice depends on the voice family. For example, the
average pitch for a standard male voice is around 120Hz, but for a
female voice, it's around 210Hz.
Values have the following meanings:
Specifies the average pitch of the speaking voice in hertz
(Hz).
x-low, low, medium, high, x-high
These values do not map to absolute frequencies since these
values depend on the voice family. User agents should map these
values to appropriate frequencies based on the voice family and
user environment. However, user agents must map these values in
order (i.e., 'x-low' is a lower frequency than 'low', etc.).
'pitch-range'
Value: | inherit
Initial: 50
Applies to: all elements
Inherited: yes
Percentages: N/A
Media: aural
Computed value: as specified
Specifies variation in average pitch. The perceived pitch of a human
voice is determined by the fundamental frequency and typically has a
value of 120Hz for a male voice and 210Hz for a female voice. Human
languages are spoken with varying inflection and pitch; these
variations convey additional meaning and emphasis. Thus, a highly
animated voice, i.e., one that is heavily inflected, displays a high
pitch range. This property specifies the range over which these
variations occur, i.e., how much the fundamental frequency may deviate
from the average pitch.
Values have the following meanings:
A value between '0' and '100'. A pitch range of '0' produces a
flat, monotonic voice. A pitch range of 50 produces normal
inflection. Pitch ranges greater than 50 produce animated
voices.
'stress'
Value: | inherit
Initial: 50
Applies to: all elements
Inherited: yes
Percentages: N/A
Media: aural
Computed value: as specified
Specifies the height of "local peaks" in the intonation contour of a
voice. For example, English is a stressed language, and different
parts of a sentence are assigned primary, secondary, or tertiary
stress. The value of 'stress' controls the amount of inflection that
results from these stress markers. This property is a companion to the
'pitch-range' property and is provided to allow developers to exploit
higher-end auditory displays.
Values have the following meanings:
A value, between '0' and '100'. The meaning of values depends
on the language being spoken. For example, a level of '50' for
a standard, English-speaking male voice (average pitch =
122Hz), speaking with normal intonation and emphasis would have
a different meaning than '50' for an Italian voice.
'richness'
Value: | inherit
Initial: 50
Applies to: all elements
Inherited: yes
Percentages: N/A
Media: aural
Computed value: as specified
Specifies the richness, or brightness, of the speaking voice. A rich
voice will "carry" in a large room, a smooth voice will not. (The term
"smooth" refers to how the wave form looks when drawn.)
Values have the following meanings:
A value between '0' and '100'. The higher the value, the more
the voice will carry. A lower value will produce a soft,
mellifluous voice.
A.10 Speech properties: 'speak-punctuation' and 'speak-numeral'
An additional speech property, 'speak-header', is described below.
'speak-punctuation'
Value: code | none | inherit
Initial: none
Applies to: all elements
Inherited: yes
Percentages: N/A
Media: aural
Computed value: as specified
This property specifies how punctuation is spoken. Values have the
following meanings:
code
Punctuation such as semicolons, braces, and so on are to be
spoken literally.
none
Punctuation is not to be spoken, but instead rendered naturally
as various pauses.
'speak-numeral'
Value: digits | continuous | inherit
Initial: continuous
Applies to: all elements
Inherited: yes
Percentages: N/A
Media: aural
Computed value: as specified
This property controls how numerals are spoken. Values have the
following meanings:
digits
Speak the numeral as individual digits. Thus, "237" is spoken
"Two Three Seven".
continuous
Speak the numeral as a full number. Thus, "237" is spoken "Two
hundred thirty seven". Word representations are
language-dependent.
A.11 Audio rendering of tables
When a table is spoken by a speech generator, the relation between the
data cells and the header cells must be expressed in a different way
than by horizontal and vertical alignment. Some speech browsers may
allow a user to move around in the 2-dimensional space, thus giving
them the opportunity to map out the spatially represented relations.
When that is not possible, the style sheet must specify at which
points the headers are spoken.
A.11.1 Speaking headers: the 'speak-header' property
'speak-header'
Value: once | always | inherit
Initial: once
Applies to: elements that have table header information
Inherited: yes
Percentages: N/A
Media: aural
Computed value: as specified
This property specifies whether table headers are spoken before every
cell, or only before a cell when that cell is associated with a
different header than the previous cell. Values have the following
meanings:
once
The header is spoken one time, before a series of cells.
always
The header is spoken before every pertinent cell.
Each document language may have different mechanisms that allow
authors to specify headers. For example, in HTML 4 ([HTML4]), it is
possible to specify header information with three different attributes
("headers", "scope", and "axis"), and the specification gives an
algorithm for determining header information when these attributes
have not been specified.
Image of a table created in MS Word [D]
Image of a table with header cells ("San Jose" and "Seattle") that are
not in the same column or row as the data they apply to.
This HTML example presents the money spent on meals, hotels and
transport in two locations (San Jose and Seattle) for successive days.
Conceptually, you can think of the table in terms of an n-dimensional
space. The headers of this space are: location, day, category and
subtotal. Some cells define marks along an axis while others give
money spent at points within this space. The markup for this table is:

Travel Expense Report

Meals

Hotels

Transport

subtotal

San Jose

25-Aug-97

37.74

112.00

45.00

26-Aug-97

27.28

112.00

45.00

subtotal

65.02

224.00

90.00

379.02

Seattle

27-Aug-97

96.25

109.00

36.00

28-Aug-97

35.00

109.00

36.00

subtotal

131.25

218.00

72.00

421.25

Totals

196.27

442.00

162.00

800.27

By providing the data model in this way, authors make it possible for
speech enabled-browsers to explore the table in rich ways, e.g., each
cell could be spoken as a list, repeating the applicable headers
before each data cell:
San Jose, 25-Aug-97, Meals: 37.74
San Jose, 25-Aug-97, Hotels: 112.00
San Jose, 25-Aug-97, Transport: 45.00
...
The browser could also speak the headers only when they change:
San Jose, 25-Aug-97, Meals: 37.74
Hotels: 112.00
Transport: 45.00
26-Aug-97, Meals: 27.28
Hotels: 112.00
...
A.12 Sample style sheet for HTML
This style sheet describes a possible rendering of HTML 4:
@media aural {
h1, h2, h3,
h4, h5, h6 { voice-family: paul, male; stress: 20; richness: 90 }
h1 { pitch: x-low; pitch-range: 90 }
h2 { pitch: x-low; pitch-range: 80 }
h3 { pitch: low; pitch-range: 70 }
h4 { pitch: medium; pitch-range: 60 }
h5 { pitch: medium; pitch-range: 50 }
h6 { pitch: medium; pitch-range: 40 }
li, dt, dd { pitch: medium; richness: 60 }
dt { stress: 80 }
pre, code, tt { pitch: medium; pitch-range: 0; stress: 0; richness: 80 }
em { pitch: medium; pitch-range: 60; stress: 60; richness: 50 }
strong { pitch: medium; pitch-range: 60; stress: 90; richness: 90 }
dfn { pitch: high; pitch-range: 60; stress: 60 }
s, strike { richness: 0 }
i { pitch: medium; pitch-range: 60; stress: 60; richness: 50 }
b { pitch: medium; pitch-range: 60; stress: 90; richness: 90 }
u { richness: 0 }
a:link { voice-family: harry, male }
a:visited { voice-family: betty, female }
a:active { voice-family: betty, female; pitch-range: 80; pitch: x-high }
}
A.13 Emacspeak
For information, here is the list of properties implemented by
Emacspeak, a speech subsystem for the Emacs editor.
* voice-family
* stress (but with a different range of values)
* richness (but with a different range of values)
* pitch (but with differently named values)
* pitch-range (but with a different range of values)
(We thank T. V. Raman for the information about implementation status
of aural properties.)
_________________________________________________________________
_________________________________________________________________
Appendix B. Bibliography
Contents
* B.1 Normative references
* B.2 Informative references
B.1 Normative references
[COLORIMETRY]
"Colorimetry", Third Edition, Commission Internationale de
l'Eclairage, CIE Publication 15:2004, ISBN 3-901-906-33-9.
Available at http://www.cie.co.at/publ/abst/15-2004.html
[FLEX]
"Flex: The Lexical Scanner Generator", Version 2.3.7, ISBN
1882114213.
[HTML4]
"HTML 4.01 Specification", D. Raggett, A. Le Hors, I. Jacobs,
24 December 1999.
The latest version of the specification is available at
http://www.w3.org/TR/html4/. The Recommendation defines three
document type definitions: Strict, Transitional, and Frameset,
all reachable from the Recommendation.
[ICC42]
Specification ICC.1:2004-10 (Profile version 4.2.0.0) Image
technology colour management - Architecture, profile format,
and data structure.
Available at http://www.color.org/icc_specs2.html
[ISO8879]
"ISO 8879:1986(E): Information processing - Text and Office
Systems - Standard Generalized Markup Language (SGML)",
International Organization for Standardization (ISO), 15
October 1986.
[ISO10646]
"Information Technology - Universal Multiple- Octet Coded
Character Set (UCS) - Part 1: Architecture and Basic
Multilingual Plane", ISO/IEC 10646-1:2003. Useful roadmap of
the BMP and plane 1 documents show which scripts sit at which
numeric ranges.
[PNG]
"Portable Network Graphics (PNG) Specification (Second
Edition)", David Duce, ed., 10 November 2003.
Available at http://www.w3.org/TR/PNG/.
[RFC3986]
"Uniform Resource Identifier (URI): Generic Syntax," T.
Berners-Lee, R. Fielding, L. Masinter, January 2005.
Available at http://www.ietf.org/rfc/rfc3986.
[RFC2045]
"Multipurpose Internet Mail Extensions (MIME) Part One: Format
of Internet Message Bodies", N. Freed and N. Borenstein,
November 1996.
Available at http://www.ietf.org/rfc/rfc2045.txt. Note that
this RFC obsoletes RFC1521, RFC1522, and RFC1590.
[RFC2616]
"HTTP Version 1.1 ", R. Fielding, J. Gettys, J. Mogul, et al.,
June 1999.
Available at http://www.ietf.org/rfc/rfc2616.txt.
[RFC2119]
"Key words for use in RFCs to Indicate Requirement Levels", S.
Bradner, March 1997.
Available at http://www.ietf.org/rfc/rfc2119.txt.
[RFC2318]
"The text/css Media Type", H. Lie, B. Bos, C. Lilley, March
1998.
Available at http://www.ietf.org/rfc/rfc2318.txt.
[SRGB]
IEC 61966-2-1 (1999-10) - "Multimedia systems and equipment -
Colour measurement and management - Part 2-1: Colour management
- Default RGB colour space - sRGB, ISBN: 2-8318-4989-6 ?ICS
codes: 33.160.60, 37.080 ?TC 100 ?51 pp.
Available at
http://domino.iec.ch/webstore/webstore.nsf/artnum/025408
[UAAG10]
"User Agent Accessibility Guidelines 1.0." Ian Jacobs, Jon
Gunderson, Eric Hansen (editors). 17 December 2002.
Available at http://www.w3.org/TR/2002/REC-UAAG10-20021217
[UNICODE]
The Unicode Consortium. The Unicode Standard, Version 4.1.0,
defined by: The Unicode Standard, Version 4.0 (Boston, MA,
Addison-Wesley, 2003. ISBN 0-321-18578-1), as amended by
Unicode 4.0.1 and Unicode 4.1.0 and as updated from time to
time by the publication of new versions. (See
http://www.unicode.org/unicode/standard/versions/ for the
latest version and additional information on versions of the
standard and of the Unicode Character Database).
Available at http://www.unicode.org/versions/Unicode4.1.0/
[XML10]
"Extensible Markup Language (XML) 1.0 (third edition)" T. Bray,
J. Paoli, C.M. Sperberg-McQueen, Eve Maler, Fran~s Yergeau,
editors, 4 February 2004.
Available at http://www.w3.org/TR/REC-xml/.
[YACC]
"YACC - Yet another compiler compiler", S. C. Johnson,
Technical Report, Murray Hill, 1975.
B.2 Informative references
[CHARSETS]
Registered charset values. Download a list of registered
charset values from
http://www.iana.org/assignments/character-sets.
[CSS1]
"Cascading Style Sheets, level 1", H. W. Lie and B. Bos, 17
December 1996, revised 11 January 1999
The latest version is available at
http://www.w3.org/TR/REC-CSS1
[CSS2]
"Cascading Style Sheets, level 2, CSS2 Specification", B. Bos,
H. W. Lie, C. Lilley and I. Jacobs, 12 May 1998,
The latest version is available at
http://www.w3.org/TR/REC-CSS2/
[CSS3COLOR]
"CSS3 Color Module," Tantek ?lik, Chris Lilley, 14 May 2003,
W3C Candidate Recommendation. Available at
http://www.w3.org/TR/2003/CR-css3-color-20030514/
[CSS3LIST]
"CSS3 module: lists," Tantek ?lik, Ian Hickson, 7 November
2002, W3C working draft (work in progress). Available at
http://www.w3.org/TR/2002/WD-css3-lists-20021107
[CSS3SEL]
"Selectors", D. Glazman, T. ?lik, I. Hickson, 13 November 2001
Available at
http://www.w3.org/TR/2001/CR-css3-selectors-20011113
[CSS3SPEECH]
"CSS3 Speech Module", David Raggett, Daniel Glazman, Claudio
Santambrogio, 14 May 2003, W3C Working Draft (work in
progress). Available at
http://www.w3.org/TR/2004/WD-css3-speech-20041216
[DOM]
"Document Object Model Specification", L. Wood, A. Le Hors, 9
October 1997.
Available at http://www.w3.org/TR/WD-DOM/
[DOM-LEVEL-3-CORE]
"Document Object Model (DOM) Level 3 Core Specification", A. Le
Hors, P. Le H~ret, et al. (eds.), 7 April 2004, W3C
Recommendation. Available at
http://www.w3.org/TR/2004/REC-DOM-Level-3-Core-20040407/.
[MATH20]
"Mathematical Markup Language (MathML) Version 2.0", D.
Carlisle, P. Ion, R. Miner, N. Poppelier, 21 February 2001
Available at http://www.w3.org/TR/2001/REC-MathML2-20010221
[P3P]
"The Platform for Privacy Preferences 1.0 (P3P1.0)
Specification", L. Cranor, M. Langheinrich, M. Marchiori, M.
Presler-Marshall, J. Reagle, 16 April 2002
Available at http://www.w3.org/TR/2002/REC-P3P-20020416
[RFC3066]
"Tags for the Identification of Languages", H. Alvestrand,
January 2001.
Available at http://www.ietf.org/rfc/rfc3066.txt.
[SVG11]
"Scalable Vector Graphics (SVG) 1.1 Specification", J.
Ferraiolo, et.al. 14 January 2003
Available at http://www.w3.org/TR/2003/REC-SVG11-20030114
[WAI-PAGEAUTH]
"Web Content Accessibility Guidelines", W. Chisholm, G.
Vanderheiden, I. Jacobs eds.
Available at: http://www.w3.org/TR/WAI-WEBCONTENT/.
[XHTML]
"XHTML 1.0 The Extensible HyperText Markup Language", various
authors,
Available at: http://www.w3.org/TR/xhtml1/.
[XMLID]
"xml:id Version 1.0", J. Marsh, D. Veillard N. Walsh, 9
November 2004, W3C working draft (work in progress). Available
at: http://www.w3.org/TR/2004/WD-xml-id-20041109/.
[XMLNAMESPACES]
"Namespaces in XML", T. Bray, D. Hollander, A. Layman,
Available at: http://www.w3.org/TR/1999/REC-xml-names-19990114.
_________________________________________________________________
_________________________________________________________________
Appendix C. Changes
Contents
* C.1 Additional property values
+ C.1.1 Section 4.3.6 Colors
+ C.1.2 Section 9.2.4 The 'display' property
+ C.1.3 Section 12.2 The 'content' property
+ C.1.4 Section 16.6 Whitespace: the 'white-space' property
+ C.1.5 Section 18.1 Cursors: the 'cursor' property
* C.2 Changes
+ C.2.1 Section 1.1 CSS 2.1 vs CSS 2
+ C.2.2 Section 1.2 Reading the specification
+ C.2.3 Section 1.3 How the specification is organized
+ C.2.4 Section 1.4.2.1 Value
+ C.2.5 Section 1.4.2.6 Media groups
+ C.2.6 Section 1.4.2.7 Computed value
+ C.2.7 Section 1.4.4 Notes and examples
+ C.2.8 Section 1.5 Acknowledgements
+ C.2.9 Section 3.2 Conformance
+ C.2.10 Section 3.3 Error Conditions
+ C.2.11 4.1.1 Tokenization
+ C.2.12 4.1.3 Characters and case
+ C.2.13 Section 4.2 Rules for handling parsing errors
+ C.2.14 Section 4.3 Values
+ C.2.15 Section 4.3.2 Lengths
+ C.2.16 Section 4.3.4 URLs and URIs
+ C.2.17 Section 4.3.6 Colors
+ C.2.18 Section 4.3.8 Unsupported Values
+ C.2.19 Section 4.4 CSS style sheet representation
+ C.2.20 Section 5.8.1 Matching attributes and attribute values
+ C.2.21 Section 5.8.3 Class selectors
+ C.2.22 Section 5.9 ID selectors
+ C.2.23 Section 5.10 Pseudo-elements and pseudo-classes
+ C.2.24 5.11.2 The link pseudo-classes: :link and :visited
+ C.2.25 Section 5.11.4 The language pseudo-class: :lang
+ C.2.26 Section 5.12.1 The :first-line pseudo-element
+ C.2.27 Section 5.12.2 The :first-letter pseudo-element
+ C.2.28 Section 6.1 Specified, computed, and actual values
+ C.2.29 Section 6.4.1 Cascading order
+ C.2.30 Section 6.4.3 Calculating a selector's specificity
+ C.2.31 Section 6.4.4 Precedence of non-CSS presentational
hints
+ C.2.32 Section 7.3 Recognized Media Types
+ C.2.33 Section 7.3.1 Media Groups
+ C.2.34 Section 8.3 Margin properties
+ C.2.35 Section 8.3.1 Collapsing margins
+ C.2.36 Section 8.4 Padding properties
+ C.2.37 Section 8.5.2 Border color
+ C.2.38 Section 8.5.3 Border style
+ C.2.39 Section 8.6 The box model for inline elements in
bidirection context
+ C.2.40 Section 9.1.2 Containing blocks
+ C.2.41 Section 9.2.1.1 Anonymous block boxes
+ C.2.42 Section 9.2.2.1 Anonymous inline boxes
+ C.2.43 Section 9.2.3 Run-in boxes
+ C.2.44 Section 9.2.4 The 'display' property
+ C.2.45 Section 9.3.1 Choosing a positioning scheme
+ C.2.46 Section 9.3.2 Box offsets
+ C.2.47 Section 9.4.1 Block formatting contexts
+ C.2.48 Section 9.4.2 Inline formatting context
+ C.2.49 Section 9.4.3 Relative positioning
+ C.2.50 Section 9.5 Floats
+ C.2.51 Section 9.5.1 Positioning the float
+ C.2.52 Section 9.5.2 Controlling flow next to floats
+ C.2.53 Section 9.7 Relationships between 'display',
'position', and 'float'
+ C.2.54 Section 9.9 Layered presentation
+ C.2.55 Section 9.10 Text direction
+ C.2.56 Chapter Visual formatting model details
+ C.2.57 Section 10.1 Definition of "containing block"
+ C.2.58 Section 10.2 Content width
+ C.2.59 Section 10.3 Calculating widths and margins
+ C.2.60 Section 10.3.2 Inline, replaced elements
+ C.2.61 Section 10.3.3 Block-level, non-replaced elements in
normal flow
+ C.2.62 Section 10.3.4 Block-level, replaced elements in
normal flow
+ C.2.63 Section 10.3.5 Floating, non-replaced elements
+ C.2.64 Section 10.3.6 Floating, replaced elements
+ C.2.65 Section 10.3.7 Absolutely positioned, non-replaced
elements
+ C.2.66 Section 10.3.8 Absolutely positioned, replaced
elements
+ C.2.67 Section 10.4 Minimum and maximum widths
+ C.2.68 Section 10.5 Content height
+ C.2.69 Section 10.6 Calculating heights and margins
+ C.2.70 Section 10.6.1 Inline, non-replaced elements
+ C.2.71 Section 10.6.2 Inline replaced elements, block-level
replaced elements in normal flow, 'inline-block' replaced
elements in normal flow and floating replaced elements
+ C.2.72 Section 10.6.3 Block-level non-replaced elements in
normal flow when 'overflow' computes to 'visible'
+ C.2.73 Section 10.6.4 Absolutely positioned, non-replaced
elements
+ C.2.74 Section 10.6.5 Absolutely positioned, replaced
elements
+ C.2.75 Section 10.7 Minimum and maximum heights
+ C.2.76 Section 10.8 Line height calculations
+ C.2.77 Section 10.8.1 Leading and half-leading
+ C.2.78 Section 11.1 Overflow and clipping
+ C.2.79 Section 11.1.1 Overflow
+ C.2.80 Section 11.1.2 Clipping: the 'clip' property
+ C.2.81 Section 11.2 Visibility
+ C.2.82 Chapter 12 Generated content, automatic numbering, and
lists
+ C.2.83 Section 12.1 The :before and :after pseudo-elements
+ C.2.84 Section 12.2 The 'content' property
+ C.2.85 Section 12.3.2 Inserting quotes with the 'content'
property
+ C.2.86 Section 12.4 Automatic counters and numbering
+ C.2.87 Section 12.4.1 Nested counters and scope
+ C.2.88 Section 12.5 Lists
+ C.2.89 Section 12.5.1 Lists
+ C.2.90 Chapter 13 Paged media
+ C.2.91 Section 13.2.2 Page selectors
+ C.2.92 Section 13.3.1 Page break properties
+ C.2.93 Section 13.3.3 Allowed page breaks
+ C.2.94 Section 14.2.1 Background properties
+ C.2.95 Section 14.3 Gamma correction
+ C.2.96 Chapter 15 Fonts
+ C.2.97 Section 15.2 Font matching algorithm
+ C.2.98 Section 15.2.2 Font family
+ C.2.99 Section 15.5 Small-caps
+ C.2.100 Section 15.6 Font boldness
+ C.2.101 Section 15.7 Font size
+ C.2.102 Chapter 16 Text
+ C.2.103 Section 16.2 Alignment
+ C.2.104 Section 16.3.1 Underlining, over lining, striking,
and blinking
+ C.2.105 Section 16.4 Letter and word spacing
+ C.2.106 Section 16.5 Capitalization
+ C.2.107 Section 16.6 Whitespace
+ C.2.108 Chapter 17 Tables
+ C.2.109 17.2 The CSS table model
+ C.2.110 Section 17.2.1 Anonymous table objects
+ C.2.111 Section 17.4 Tables in the visual formatting model
+ C.2.112 Section 17.4.1 Caption position and alignment
+ C.2.113 Section 17.5 Visual layout of table contents
+ C.2.114 Section 17.5.1 Table layers and transparency
+ C.2.115 Section 17.5.2.1 Fixed table layout
+ C.2.116 Section 17.5.2.2 Automatic table layout
+ C.2.117 Section 17.5.3 Table height algorithms
+ C.2.118 17.5.4 Horizontal alignment in a column
+ C.2.119 Section 17.6 Borders
+ C.2.120 Section 17.6.1 The separated borders model
+ C.2.121 Section 17.6.1.1 Borders and Backgrounds around empty
cells
+ C.2.122 Section 17.6.2 The collapsing border model
+ C.2.123 Section 17.6.2.1 Border conflict resolution
+ C.2.124 Section 18.4 Dynamic outlines
+ C.2.125 Chapter 12 Generated content, automatic numbering,
and lists
+ C.2.126 Appendix A. Aural style sheets
+ C.2.127 Appendix A Section 5 Pause properties
+ C.2.128 Appendix A Section 6 Cue properties
+ C.2.129 Appendix A Section 7 Mixing properties
+ C.2.130 Appendix B Bibliography
+ C.2.131 Other
* C.3 Errors
+ C.3.1 Shorthand properties
+ C.3.2 Applies to
+ C.3.3 Section 4.1.1 (and G2)
+ C.3.4 Section 4.1.3 Characters and case
+ C.3.5 Section 4.3 (Double sign problem)
+ C.3.6 Section 4.3.2 Lengths
+ C.3.7 Section 4.3.3 Percentages
+ C.3.8 Section 4.3.4 URLs and URIs
+ C.3.9 Section 4.3.5 Counters
+ C.3.10 Section 4.3.6
+ C.3.11 Section 4.3.7 Strings
+ C.3.12 Section 5.10 Pseudo-elements and pseudo-classes
+ C.3.13 Section 6.4 The cascade
+ C.3.14 Section 8.1 Box Dimensions
+ C.3.15 8.2 Example of margins, padding, and borders
+ C.3.16 Section 8.5.4 Border shorthand properties
+ C.3.17 Section 9.2.1 Block-level elements and block boxes
+ C.3.18 Section 9.3.1 Choosing a positioning scheme
+ C.3.19 Section 9.3.2 Box offsets
+ C.3.20 Section 9.4.1 Block formatting contexts
+ C.3.21 Section 9.4.2 Inline formatting context
+ C.3.22 Section 9.4.3 Relative positioning
+ C.3.23 Section 9.5 Floats
+ C.3.24 Section 9.5.1 Positioning the float
+ C.3.25 Section 9.5.2 Controlling flow next to floats
+ C.3.26 Section 9.6 Absolute positioning
+ C.3.27 Section 9.7 Relationships between 'display',
'position', and 'float'
+ C.3.28 Section 9.10 Text direction
+ C.3.29 Section 10.1 Definition of "containing block"
+ C.3.30 Section 10.3.3
+ C.3.31 Section 10.4 Minimum and maximum widths
+ C.3.32 Section 10.6.3 Block-level non-replaced elements in
normal flow when 'overflow' computes to 'visible'
+ C.3.33 Section 10.7 Minimum and maximum heights
+ C.3.34 Section 11.1.1 Overflow
+ C.3.35 Section 11.1.2 Clipping: the 'clip' property
+ C.3.36 Section 11.2 Visibility
+ C.3.37 12.6.2 Lists
+ C.3.38 14.2 The background
+ C.3.39 14.2.1 Background properties
+ C.3.40 Section 15.2
+ C.3.41 Section 15.7 Font size
+ C.3.42 Section 16.1 Indentation
+ C.3.43 Section 16.2 Alignment
+ C.3.44 Section 17.2 The CSS table model
+ C.3.45 Section 17.2.1 Anonymous table objects
+ C.3.46 Section 17.4 Tables in the visual formatting model
+ C.3.47 Section 17.5 Visual layout of table contents
+ C.3.48 Section 17.5.1 Table layers and transparency
+ C.3.49 Section 17.6.1 The separated borders model
+ C.3.50 Section 18.2
* C.4 Clarifications
+ C.4.1 Section 2.1 A brief CSS 2.1 tutorial for HTML
+ C.4.2 Section 2.2 A brief CSS 2.1 tutorial for XML
+ C.4.3 Section 3.1 Definitions
+ C.4.4 Section 4.1 Syntax
+ C.4.5 Section 4.1.1 Tokenization
+ C.4.6 Section 4.1.3 Characters and case
+ C.4.7 Section 4.1.7 Rule sets, declaration blocks, and
selectors
+ C.4.8 Section 4.2 Rules for handling parsing errors
+ C.4.9 Section 4.3.1 Integers and real numbers
+ C.4.10 Section 4.3.2 Lengths
+ C.4.11 Section 4.3.4 URLs and URIs
+ C.4.12 Section 5.1 Pattern matching
+ C.4.13 Section 5.7 Adjacent sibling selectors
+ C.4.14 Section 5.8.2 Default attribute values in DTDs
+ C.4.15 Section 5.9 ID selectors
+ C.4.16 Section 5.11.3 The dynamic pseudo-classes: :hover,
:active, and :focus
+ C.4.17 Section 5.11.4 The language pseudo-class: :lang
+ C.4.18 Section 5.12.2 The :first-letter pseudo-element
+ C.4.19 Section 6.2 Inheritance
+ C.4.20 Section 6.2.1 The 'inherit' value
+ C.4.21 Section 6.3 The @import rule
+ C.4.22 6.4 The Cascade
+ C.4.23 6.4.1 Cascading order
+ C.4.24 Section 6.4.3 Calculating a selector's specificity
+ C.4.25 Section 7.2.1 The @media rule
+ C.4.26 Section 7.3 Recognized media types
+ C.4.27 Section 7.3.1 Media groups
+ C.4.28 Section 8.1 Box dimensions
+ C.4.29 Section 8.3 Margin properties
+ C.4.30 Section 8.3.1
+ C.4.31 Section 8.5.3 Border style
+ C.4.32 Section 9.1.1 The viewport
+ C.4.33 Section 9.3.1 Choosing a positioning scheme
+ C.4.34 Section 9.3.2 Box offsets
+ C.4.35 Section 9.4.2 Inline formatting context
+ C.4.36 Section 9.4.3 Relative positioning
+ C.4.37 Section 9.5 Floats
+ C.4.38 Section 9.5.1 Positioning the float
+ C.4.39 Section 9.5.2 Controlling flow next to floats
+ C.4.40 Section 9.8 Comparison of normal flow, floats, and
absolute positioning
+ C.4.41 Section 10.1 Definition of "containing block"
+ C.4.42 Section 10.2 Content width
+ C.4.43 Section 10.3.3 Block-level, non-replaced elements in
normal flow
+ C.4.44 Section 10.4 Minimum and maximum widths
+ C.4.45 Section 10.6 Calculating heights and margins
+ C.4.46 Section 10.7 Minimum and maximum heights
+ C.4.47 Section 10.8 Line height calculations
+ C.4.48 Section 11.1 Overflow and clipping
+ C.4.49 Section 11.1.1 Overflow
+ C.4.50 Section 11.1.2 Clipping
+ C.4.51 Section 11.2 Visibility
+ C.4.52 Section 12.1 The :before and :after pseudo-elements
+ C.4.53 Section 12.2 The 'content' property
+ C.4.54 Section 12.3.2 Inserting quotes with the 'content'
property
+ C.4.55 Section 12.4 Automatic counters and numbering
+ C.4.56 Section 12.4.3 Counters in elements with 'display:
none'
+ C.4.57 Section 14.2 The background
+ C.4.58 Section 15.1 Fonts Introduction
+ C.4.59 Section 15.2 Font matching algorithm
+ C.4.60 Section 15.2.2 Font family
+ C.4.61 Section 15.3.1 Generic font families
+ C.4.62 Section 15.4 Font styling
+ C.4.63 Section 15.5 Small-caps
+ C.4.64 Section 15.6 Font boldness
+ C.4.65 Section 15.7 Font size
+ C.4.66 Section 16.1 Indentation
+ C.4.67 16.2 Alignment
+ C.4.68 Section 16.3.1 Underlining, over lining, striking, and
blinking
+ C.4.69 Section 16.5 Capitalization
+ C.4.70 Section 16.6 Whitespace
+ C.4.71 Section 17.1 Introduction to tables
+ C.4.72 Section 17.2 The CSS table model
+ C.4.73 Section 17.2.1 Anonymous table objects
+ C.4.74 Section 17.4 Tables in the visual formatting model
+ C.4.75 Section 17.5 Visual layout of table contents
+ C.4.76 Section 17.5.1 Table layers and transparency
+ C.4.77 Section 17.5.2 Table width algorithms
+ C.4.78 Section 17.5.2.1 Fixed table layout
+ C.4.79 Section 17.5.2.2 Automatic table layout
+ C.4.80 Section 17.5.5 Dynamic row and column effects
+ C.4.81 17.6.1 The separated borders model
+ C.4.82 Section 17.6.2 The collapsing borders model
+ C.4.83 Section 18.2 System Colors
+ C.4.84 Section 18.4 Dynamic outlines
+ C.4.85 Section 18.4.1 Outlines and the focus
+ C.4.86 Appendix D
* C.5 Code Diffs for Appendices D and G
+ C.5.1 Appendix D Default style sheet for HTML4
+ C.5.2 Section G.1 Grammar
+ C.5.3 Section G.2 Lexical scanner
This appendix is informative, not normative.
CSS 2.1 is an updated revision of CSS2. The changes between the CSS2
specification (see [CSS2]) and this specification fall into five
groups: known errors, typographical errors, clarifications, changes
and additions. Typographical errors are not listed here.
This chapter is not a complete list of changes. Minor editorial
changes and most changes to examples are also not listed here.
C.1 Additional property values
C.1.1 Section 4.3.6 Colors
New color value: 'orange'
C.1.2 Section 9.2.4 The 'display' property
New 'display' value: 'inline-block'
C.1.3 Section 12.2 The 'content' property
New 'content' values 'none' and 'normal'. (The values 'none' and
'normal' are equivalent in CSS 2.1, but may have different functions
in CSS3.)
C.1.4 Section 16.6 Whitespace: the 'white-space' property
New 'white-space' values: 'pre-wrap' and 'pre-line'
C.1.5 Section 18.1 Cursors: the 'cursor' property
New 'cursor' value: 'progress'
C.2 Changes
C.2.1 Section 1.1 CSS 2.1 vs CSS 2
This new section is added to explain the motivation for CSS2.1 and its
relation to CSS2.
C.2.2 Section 1.2 Reading the specification
This section (formerly Section 1.1) has been marked non-normative.
C.2.3 Section 1.3 How the specification is organized
This section (formerly Section 1.2) has been marked non-normative.
C.2.4 Section 1.4.2.1 Value
This section (formerly unnumbered under 1.3.2) notes that value types
are specified in terms of tokens and that spaces may appear between
tokens in values. A note explains that spaces are required between
some tokens.
C.2.5 Section 1.4.2.6 Media groups
This section (formerly unnumbered under 1.3.2) now declares the Media
line in property definitions to be non-normative.
C.2.6 Section 1.4.2.7 Computed value
A new line is added to each property definition specifying what the
computed values are for the property. (This defines what level of
computation is done to a property value before inheritance and before
certain other calculations.)
C.2.7 Section 1.4.4 Notes and examples
This section (formerly 1.3.4) now specifies that HTML examples lacking
DOCTYPE declarations are SGML Text Entities conforming to the HTML
4.01 Strict DTD [HTML4]. The markup for many examples has been
reformulated to either include a DOCTYPE or conform to this
definition.
C.2.8 Section 1.5 Acknowledgements
This section (formerly 1.4) has been updated to reflect contributions
to CSS2.1 and has been marked non-normative.
C.2.9 Section 3.2 Conformance
Support for user style sheets is now required (in most cases), rather
than just recommended.
Support for turning of author style sheets is now required.
Application of CSS properties to form controls is explicitly
undefined. Authors are recommended to treat form control styling
capabilities in UAs as experimental.
C.2.10 Section 3.3 Error Conditions
This section changed to say that error handling is specified in most
cases.
C.2.11 4.1.1 Tokenization
Added INVALID token and rules for its definition.
An optional hyphen, "-", is now allowed at the beginning of an "ident"
for vendor extensions. (See section 4.1.2.1)
The underscore character ("_") is allowed in identifiers. The
definitions of the lexical macros "nmstart" and "nmchar" now include
it. See also section 4.1.2.1 (Vendor extensions).
The "escape" macro has been modified to allow the escaping of any
character except newlines, form feeds, and hex digits (to avoid
conflict with unicode escapes).
Modified "string1" and "string2" macros by defining allowed characters
through excluding disallowed characters. This allows invisible ASCII
characters to be included in a string.
C.2.12 4.1.3 Characters and case
Updated prose about identifiers (second bullet point) to match changes
in the tokenization (above).
Excluded null (0x0) character from CSS numerical escapes and indicate
that it is undefined in CSS2.1 what happens if such a character is
encountered.
Allowed the use of U+FFFD as a replacement for characters outside the
range allowed by Unicode.
C.2.13 Section 4.2 Rules for handling parsing errors
Defined parsing in the cases of Malformed Declarations, Unexpected End
of Stylesheet, and Unexpected End of String.
C.2.14 Section 4.3 Values
Sections 4.3.7 (Angles), 4.3.8 (Times), and 4.3.9 (Frequencies) have
been moved to the informative Appendix A.
C.2.15 Section 4.3.2 Lengths
Added a paragraph on heuristics for finding the x-height of a font.
C.2.16 Section 4.3.4 URLs and URIs
Updated URI references to RFC3986.
C.2.17 Section 4.3.6 Colors
Defined the numeric values corresponding to color keywords instead of
referencing HTML4 for those values.
UAs are now allowed to intelligently map colors outside the gamut into
the gamut instead of simply clipping them into the range of the gamut.
C.2.18 Section 4.3.8 Unsupported Values
Added this section to recommend that unsupported properties and values
be ignored as if they were invalid.
C.2.19 Section 4.4 CSS style sheet representation
Changed character encoding detection rule 2 to include a BOM and
referred to additional rules below.
Added rule 4 to provide for use of the referring style sheet or
document's character encoding.
Added rule 5 to require falling back to UTF-8.
Removed the restriction on using @charset in embedded style sheets.
Allowed a BOM to precede the @charset rule.
Added requirement that @charset rule must be a literal
'@charset"...";', not a CSS-syntax equivalent.
Added requirement to support for UTF-8 at minimum.
Specified that nay @charset rule not at the beginning of the style
sheet must be ignored.
Removed note on theoretical problem with @charset problem and
precisely defined rules for character encoding detection based on
@charset and/or BOM.
Specified that UAs must ignore style sheets in unknown encodings.
C.2.20 Section 5.8.1 Matching attributes and attribute values
RFC 3066 replaces RFC 1766.
C.2.21 Section 5.8.3 Class selectors
Class selectors are allowed for other formats than HTML.
Added a note about matching classes in formats with multiple class
attributes per element. The behavior is non-normative, because, at the
time of writing, there exist no such formats.
C.2.22 Section 5.9 ID selectors
Specified how to match elements with two or more ID attributes.
C.2.23 Section 5.10 Pseudo-elements and pseudo-classes
Removed exception for HTML UAs that allowed them (and only them) to
ignore ':first-letter' and ':first-line'.
C.2.24 5.11.2 The link pseudo-classes: :link and :visited
UAs may return a :visited link to :link status at some point. (This
was previously a note, but is now normative.)
Added a note about privacy concerns with link pseudo classes and
allowed UAs to treat :visited as :link.
C.2.25 Section 5.11.4 The language pseudo-class: :lang
The identifier C in ':lang(C)' need not be a valid language code, but
it must not be empty.
C.2.26 Section 5.12.1 The :first-line pseudo-element
':first-line' also applies to inline blocks, table captions and table
cells. Added a definition of "first formatted line" to make the rules
about which line is the first line more precise.
UAs are no longer forbidden from applying more properties than the
given list.
C.2.27 Section 5.12.2 The :first-letter pseudo-element
More precise definition of first letter. Added rules for cases where
the first letter is in an inline block or table cell. Added rules for
cases when preceding punctuation is in a different element from the
first letter itself.
UAs may apply other properties to first letters than the given list.
Unicode character classes Pi and Pf added to the definition of
punctuation.
C.2.28 Section 6.1 Specified, computed, and actual values
Redefined "computed value" and created the concept of "used value" so
that inheritance can be performed without laying out the document.
This change has the effect of allowing (requiring) percentages to be
inherited as percentages and affects many other layout calculations
throughout the spec.
Since computed value of a property can now also be a percentage. In
particular, the following properties now inherit the percentage if the
specified value is a percentage:
* background-position
* bottom, left, right, top
* height, width
* margin-bottom, margin-left, margin-right, margin-top,
* min-height, min-width
* padding-bottom, padding-left, padding-right, padding-top
* text-indent
Note that only 'text-indent' inherits by default, the others only
inherit if the 'inherit' keyword is specified.
C.2.29 Section 6.4.1 Cascading order
Changed suggestion that user be able to turn off author styles to a
requirement.
C.2.30 Section 6.4.3 Calculating a selector's specificity
The "style" attribute now has a higher specificity than any style
rule.
Pseudo-elements are now counted with elements in calculating a a
selector's specificity.
C.2.31 Section 6.4.4 Precedence of non-CSS presentational hints
"Non-CSS presentational hints" no longer exist, with the exception of
a small set of attributes in HTML.
C.2.32 Section 7.3 Recognized Media Types
Added 'speech' media type.
Marked "Media" field in property descriptions informative.
C.2.33 Section 7.3.1 Media Groups
Marked this section informative.
Added sound to 'handheld' in media type/media group table.
Changed 'tactile' to be both 'static' and 'interactive'.
C.2.34 Section 8.3 Margin properties
If the containing block's width depends on an element with percentage
margins, then the resulting layout is undefined in CSS 2.1.
C.2.35 Section 8.3.1 Collapsing margins
In the definition of "collapsing margins", added "non-empty content"
and "clearance" to the parenthetical list of things that prevent
consecutive margins from being adjoining.
Vertical margins of elements with 'overflow' other than 'visible' no
longer collapse with their in-flow children.
Defined how margins collapse through an element with adjoining top and
bottom margins.
Added that margins of the root element's box do not collapse.
More rigorously defined "adjoining" for margin collapsing.
C.2.36 Section 8.4 Padding properties
If the containing block's width depends on an element with percentage
padding, then the resulting layout is undefined in CSS 2.1.
C.2.37 Section 8.5.2 Border color
'transparent' can now be specified independently for each border side,
on par with .
C.2.38 Section 8.5.3 Border style
3D border styles ('groove', 'ridge', 'inset', 'outset') now depend on
the corresponding border-color rather than on 'color'.
C.2.39 Section 8.6 The box model for inline elements in bidirection context
Added this new section to specify layout of inline boxes when affected
by bidi.
C.2.40 Section 9.1.2 Containing blocks
Removed paragraphs about the initial containing block, as this is now
defined differently. (See changes to section 10.1.)
C.2.41 Section 9.2.1.1 Anonymous block boxes
Added a paragraph to define formatting when an inline box contains a
block box.
Specified what property values are applied to anonymous boxes.
C.2.42 Section 9.2.2.1 Anonymous inline boxes
Specified that collapsed whitespace does not generate anonymous inline
boxes.
C.2.43 Section 9.2.3 Run-in boxes
Changed run-in rules so that a) run-ins that contain blocks become
blocks b) run-ins can only run into sibling blocks and c) run-ins
cannot run into other run-ins.
C.2.44 Section 9.2.4 The 'display' property
The 'marker' and 'compact' values of the 'display' property are not
part of CSS 2.1. Text relating to these values has been removed
throughout the specification.
Defined the computed value of 'display' as the specified value except
for positioned and floating elements and for the root element. The
computed value of 'display' for these elements is defined in section
9.7 and is slightly different from the definition in CSS2.
Conforming HTML UAs are no longer allowed to ignore the 'display'
property.
C.2.45 Section 9.3.1 Choosing a positioning scheme
The 'position' property now applies to all elements, including
generated content.
The effect of relative positioning on table captions and internal
table elements is undefined in CSS 2.1.
For fixed positioning, introduced a conflict between this section and
section 10.1 rule 3. See howcome for rationale.
Forbid UAs from paginating the content of fixed boxes.
UAs are allowed to treat all values of 'position' as 'static' on the
root element.
C.2.46 Section 9.3.2 Box offsets
Defined computed values of 'top', 'right', 'bottom', 'left' based on
the value of 'position'.
Percentage offsets are no longer undefined for containing blocks
without an explicit height.
C.2.47 Section 9.4.1 Block formatting contexts
Specified that floats, absolutely positioned elements, inline-blocks,
table-cells, table-captions, and elements with 'overflow' other than
'visible' establish new block formatting contexts.
In the paragraph about the position of a box's outer edge with respect
to its containing block, except boxes that establish a new block
formatting context, as they may become narrower due to floats.
C.2.48 Section 9.4.2 Inline formatting context
Specified that the effect of 'justify' on the content of a line box
does not affect the contents of inline-table and inline-block boxes.
Empty line boxes are now required to be treated as zero-height and
ignored in margin collapsing.
C.2.49 Section 9.4.3 Relative positioning
Added several paragraphs and an example to explain exactly what the
computed values of relatively-positioned offsets are, how they affect
each other, and what happens when the positioning is overconstrained.
(These were not previously defined.)
C.2.50 Section 9.5 Floats
Floats are no longer required to have an explicit width.
Floats outside of line boxes no longer align to the bottom of the
preceding block box; it is implied that they are initially aligned
with their non-floated position.
Specified that "If a shortened line box is too small to contain any
further content, then it is shifted downward until either it fits or
there are no more floats present."
Specified that the border box of a table, block-level replaced
element, or element in the normal flow that establishes a new block
formatting context must not overlap any floats in the same block
formatting context.
C.2.51 Section 9.5.1 Positioning the float
The 'float' property now also applies to :before/:after and generated
content.
UAs are now allowed to treat all values of float as 'none' on the root
element.
Added to rule 4 prose to define the position of a float when it occurs
between two collapsing margins.
C.2.52 Section 9.5.2 Controlling flow next to floats
Defined clearance to precisely detail the 'clear' property's effect on
margin collapsing and the block's cleared position.
Added note to explain effect of 'clear' on inline elements since CSS1
(but not CSS2 or CSS 2.1) allows 'clear' on inline elements.
C.2.53 Section 9.7 Relationships between 'display', 'position', and 'float'
Changed rules to convert 'display' not always to 'block', but to an
appropriate block-level display value as given by a mapping table.
Added rule 4 to convert root element's 'display' value according to
the mapping.
C.2.54 Section 9.9 Layered presentation
Specified that the background and borders of an element that forms a
stacking context are behind all of its descendants, altered stacking
context prose to be more precise, and added a normative Appendix E:
Elaborate description of Stacking Contexts to be even more precise
about the position of borders, backgrounds, and content on the z-axis.
C.2.55 Section 9.10 Text direction
Conforming UAs are now allowed to not support bidirectional text; in
this case they must ignore the 'direction' and 'unicode-bidi'
properties. However since applying bidi can have an effect even when a
document doesn't contain right-to-left characters, UAs that do support
bidi are no longer permitted to not apply the algorithm just because
the document lacks right-to-left characters.
Added a paragraph to define precisely how the Unicode bidirectional
algorithm applies to text in the CSS formatting model and how the CSS
'direction' property on blocks maps into the algorithm.
Conforming HTML UAs are no longer exempt from supporting 'direction'
and 'unicode-bidi'.
C.2.56 Chapter Visual formatting model details
Updated prose to use the terms "specified", "computed" and "used" as
appropriate when referencing values. This affects many calculations in
this section. (See changes to section 6.1.)
C.2.57 Section 10.1 Definition of "containing block"
In rule 1, defined the initial containing block as the viewport for
continuous media and the page area for paged media. (It was previously
undefined.)
In rule 2, defined the page area as the containing block for fixed
positioned elements in paged media.
In rule 4.1, when the containing block of an absolutely-positioned
element is formed by an inline-level element, it is now formed by that
element's padding edges, not its content edges.
In rule 4, changed the containing block for absolutely positioned
elements with only statically positioned elements from the root's
content box to the initial containing block.
Specified the positioning and breaking behavior of
absolutely-positioned elements in paged media.
C.2.58 Section 10.2 Content width
Declared that if the containing block's width depends on an element's
percentage width, then the resulting layout is undefined in CSS 2.1.
C.2.59 Section 10.3 Calculating widths and margins
The computed values of 'left' and 'right' for are now defined in
section 9.3.2. The value 'auto' does not always compute to zero.
Added sections 10.3.9 and 10.3.10 to define calculations for inline
blocks.
C.2.60 Section 10.3.2 Inline, replaced elements
The sizing algorithm for replaced elements now takes into account and
attempts to preserve the replaced content's intrinsic ratio. Sizing of
replaced elements with percentage intrinsic sizes and without
intrinsic sizes is now also defined.
C.2.61 Section 10.3.3 Block-level, non-replaced elements in normal flow
Added the vertical scrollbar width to the horizontal constraint
equation.
Specified that a computed total of the width, padding, and borders
that is greater than the containing block width causes auto margins to
be treated as zero in the rest of the rules. This avoids 'auto'
margins being negative on the start edge.
C.2.62 Section 10.3.4 Block-level, replaced elements in normal flow
Applied changes to section 10.3.2 and section 10.3.3 to block-level
replaced elements in normal flow by referring to the calculations in
those sections.
C.2.63 Section 10.3.5 Floating, non-replaced elements
Defined computations for 'auto' width floats as shrink-to-fit. (Floats
were previously required to have fixed widths.)
C.2.64 Section 10.3.6 Floating, replaced elements
Applied changes to section 10.3.2 to this section by referencing it
for 'auto' width calculations.
C.2.65 Section 10.3.7 Absolutely positioned, non-replaced elements
Defined the static position of an element more precisely.
Added vertical scrollbar width to the horizontal restraint equation.
Rewrote constraint rules.
C.2.66 Section 10.3.8 Absolutely positioned, replaced elements
In rule 1, applied sizing rules from section 10.3.2.
In rule 2 (formerly rules 2 and 3), referred to new definition of
'static position' in section 10.3.7.
In rule 4 (formerly rule 5), prevented 'auto' left and right margins
in resulting in a negative margin on the start edge.
C.2.67 Section 10.4 Minimum and maximum widths
Specified that if the containing block's width is negative, the used
value of a percentage min/max width is zero.
Specified that if the min/max width is specified in percentages and
the containing block's width depends on this element's width, then the
resulting layout is undefined in CSS 2.1.
The UA is no longer allowed to select an aribtrary minimum width.
The used width of replaced elements with an intrinsic ratio and both
'width' and 'height' specified as 'auto' is now calculated according
to a table designed to preserve the intrinsic ratio as much as
possible within the given constraints.
C.2.68 Section 10.5 Content height
Removed mention of 'line-height' for inline elements since their
content box height no longer depends on 'line-height'.
Percentage heights on absolutely-positioned elements are no longer
treated as 'auto' when the containing block's height is not explicitly
specified. Added a note to explain why this is possible.
Specified that a percentage height on the root element is relative to
the initial containing block.
C.2.69 Section 10.6 Calculating heights and margins
The computed values of 'top' and 'bottom' for are now defined in
section 9.3.2. The value 'auto' does not always compute to zero.
Added section 10.6.6 to cover cases that are no longer covered under
the previous sections.
Added section 10.6.7 to define 'auto' heights for block formatting
context roots. (Unlike other block boxes, the height of these boxes
increases to accomodate any normal-flow descendant floats.)
C.2.70 Section 10.6.1 Inline, non-replaced elements
The height of an inline box is no longer given by the 'line-height'
property and is now undefined. This section now suggests that the
height of the box can be based on the font.
C.2.71 Section 10.6.2 Inline replaced elements, block-level replaced elements
in normal flow, 'inline-block' replaced elements in normal flow and floating
replaced elements
The sizing algorithm for replaced elements now takes into account and
attempts to preserve the replaced content's intrinsic ratio. Sizing of
replaced elements with percentage intrinsic sizes and without
intrinsic sizes is now also defined.
Specified that for inline elements, the margin box is used when
calculating the height of the line box.
C.2.72 Section 10.6.3 Block-level non-replaced elements in normal flow when
'overflow' computes to 'visible'
This section now only applies to elements whose 'overflow' value
computes to 'visible'; elements with other values of 'overflow' are
discussed in the new section 10.6.7 ('Auto' heights for block
formatting context roots).
C.2.73 Section 10.6.4 Absolutely positioned, non-replaced elements
Defined the static position of an element more precisely.
Added vertical scrollbar width to the horizontal restraint equation.
Rewrote constraint rules.
C.2.74 Section 10.6.5 Absolutely positioned, replaced elements
In rule 1, applied sizing rules from section 10.6.2.
C.2.75 Section 10.7 Minimum and maximum heights
Percentage min/max heights on absolutely-positioned elements are no
longer treated as '0'/'none' when the containing block's height is not
explicitly specified. However if the containing block's width depends
on an element's percentage width, then the resulting layout is
undefined in CSS 2.1.
The used width of replaced elements with an intrinsic ratio and both
'width' and 'height' specified as 'auto' is now calculated according
to a table designed to preserve the intrinsic ratio as much as
possible within the given constraints.
C.2.76 Section 10.8 Line height calculations
Added rule 4 to specify that the height of the line box must be at
least as much as that specified by the 'line-height' property on the
this block.
C.2.77 Section 10.8.1 Leading and half-leading
UAs are no longer permitted to clip content to the line box, and are
instead asked to render overlapping boxes in document order.
'line-height' set on a block no longer specifies the minimal height of
each inline box; instead it specifies the minimal height of each line
box. The exact effect of this requirement is expressed in terms of
struts; it is affected by vertical-alignment.
Adjusted text to reflect that the content box height of an inline is
no longer dictated by the 'line-height' property.
Since the content box is now defined by the font and not by the
line-height, 'text-top' and 'text-bottom' refer to the content area
instead of the font.
Defined 'top' and 'bottom' alignment in terms of aligned subtrees to
take into account any protruding descendants.
Defined the baseline of inline tables and inline blocks.
C.2.78 Section 11.1 Overflow and clipping
Specified that 'overflow' clips to the padding edge.
C.2.79 Section 11.1.1 Overflow
'projection' media are no longer permitted to print overflowing
content for 'overflow: scroll'. 'print' media now may, as opposed to
should.
UAs are now required to apply the 'overflow' property set on the root
element to the viewport. Additionally, HTML UAs must use the
'overflow' property on the HTML BODY element instead if the root
element's 'overflow' value is 'visible'.
Specified placement of scrollbar in the box model.
C.2.80 Section 11.1.2 Clipping: the 'clip' property
The 'clip' property now applies only to absolutely positioned
elements. Furthermore, it applies to those elements even when their
'overflow' is 'visible'.
The default value of 'clip', 'auto', now indiciates no clipping rather
than clipping to the element's border box.
Values of "rect()" should be separated by commas. UAs are required to
support this syntax, but may also support a space-separated syntax
since CSS2 was not clear about this.
While CSS2 specified that values of "rect()" give offsets from the
respective sides of the box, current implementations interpret values
with respect to the top and left edges for all four values (top,
right, bottom, and left). This is now the specified interpretation.
C.2.81 Section 11.2 Visibility
The 'visibility' property is now defined to inherit, and descendant
elements can override an ancestor's hidden visibility.
C.2.82 Chapter 12 Generated content, automatic numbering, and lists
Moved all discussion of aural rendering to Appendix A.
C.2.83 Section 12.1 The :before and :after pseudo-elements
Removed restrictions on which properties and property values are
allowed on ':before' and ':after' pseudo-elements.
C.2.84 Section 12.2 The 'content' property
The initial value of 'content' is now 'normal', not the empty string.
The 'content' property now distinguishes between the empty string,
which creates an empty box; and 'normal'/'none', which create no box
at all. (There is no distinction between 'normal' and 'none' in
level 2.)
A UA is now allowed to report a URI that fails to download.
Removed recommendation to authors to put rules with media-sensitive
'content' properties inside '@media'.
Whether '\A' escapes in generated content create line breaks is now
subject to the 'white-space' property.
The former section 12.3 on interaction between ':before', ':after' and
elements with 'display: compact' or 'display: run-in' has been
removed. (The interaction is already fully defined, because generated
content consists of boxes in the tree, no different from other boxes.)
C.2.85 Section 12.3.2 Inserting quotes with the 'content' property
Specified that extra 'close-quote's and 'no-close-quote's (those
without a matching 'open-quote' or 'no-open-quote') are not rendered,
and that neither 'close-quote' nor 'no-close-quote' cause the quoting
depth to be negative.
C.2.86 Section 12.4 Automatic counters and numbering
Defined what a rule with duplicate counters, such as 'counter-reset:
section 2 section', means.
C.2.87 Section 12.4.1 Nested counters and scope
The scope of a counter no longer defaults to the whole document, but
starts at the first element that uses the counter. (This affects
counters that are used without a prior 'counter-reset' to set the
scope explicitly.)
C.2.88 Section 12.5 Lists
Removed text in section 12.5 (formerly 12.6) relating to the 'marker'
display value.
Removed the 'marker-offset' property (and thus former section 12.6.1).
C.2.89 Section 12.5.1 Lists
The list styles 'hebrew', 'armenian', 'georgian', 'cjk-ideographs',
'hiragana', 'katakana', 'hiragana-iroha' and 'katakana-iroha' have
been removed due to lack of implementation experience. (They are
expected to return in the CSS3 Lists module.)
Removed the sentence that said that an unknown value for
'list-style-type' should cause the value 'decimal' to be used instead.
Instead, normal parsing rules apply and cause the rule to be ignored.
C.2.90 Chapter 13 Paged media
The 'size', 'marks', and 'page' properties are not part of CSS 2.1.
C.2.91 Section 13.2.2 Page selectors
The requirement for UA's to honor different declarations for :left,
:right, and :first pages has been softened to simplify
implementations: the page area of the :first page may be used for
:left and :right pages as well.
C.2.92 Section 13.3.1 Page break properties
UAs are now only required to apply the page break properties to
block-level elements in the normal flow of the root element, not to
other blocks.However, UAs are now permitted to apply these properties
to elements other than block-level elements.
Defined treatment of margins, borders, and padding when a page break
splits a box.
C.2.93 Section 13.3.3 Allowed page breaks
When dropping restrictions to find a page breaking opportunity, rule A
is dropped together with B and D rather than together with C.
Removed restriction on breaking within absolutely positioned boxes.
C.2.94 Section 14.2.1 Background properties
For 'background-position', the restriction that keywords cannot be
combined with percentage or length values is removed. I.e., a value
like: '25% top' is now allowed. Also, 'background-position' now
applies to all elements, not just to block-level and replaced
elements.
User agents are no longer allowed to treat a value of 'fixed' for
'background-attachment' as 'scroll'. Instead they must ignore all such
declarations as if 'fixed' were an invalid value.
C.2.95 Section 14.3 Gamma correction
The contents of this section is now a non-normative note.
C.2.96 Chapter 15 Fonts
The 'font-stretch' and 'font-size-adjust' properties have been removed
in CSS 2.1.
Font descriptors, the '@font-face' declaration, and all associated
parts of the font matching algorithm have been removed in CSS 2.1.
C.2.97 Section 15.2 Font matching algorithm
In this section (previously 15.5), in step 5 (prevously 8) of the font
matching algorith, the UA is now allowed to use multiple default
fallback fonts to find a glyph for a given character.
In the per-property rule 2, specified that if there is only a
small-caps font in a given family, then that font will be selected by
'normal'.
C.2.98 Section 15.2.2 Font family
The "missing character" glyph is no longer considered a match for the
last font in a font set, but is now considered a match for U+FFFD.
Certain punctuation characters when appearing in unquoted font family
names are now required to be escaped.
C.2.99 Section 15.5 Small-caps
The 'font-variant' property's effect is no longer restricted to
bicameral scripts.
C.2.100 Section 15.6 Font boldness
The computed value of 'font-weight' has been defined more precisely
such that the 'bolder' and 'lighter' values have an appropriate effect
when inheriting through elements with different font-families.
C.2.101 Section 15.7 Font size
Removed suggestion of 1.2 fixed ratio between keyword font sizes in
favor of notes recommending a variable ratio and a smallest font-size
no less than 9 pixels per EM unit.
Added table mapping CSS font-size keywords to HTML font size numbers.
C.2.102 Chapter 16 Text
The 'text-shadow' property is not in CSS 2.1.
C.2.103 Section 16.2 Alignment
The initial value of 'text-align' is no longer UA-defined but a
nameless value that acts as 'left' if 'direction' is 'ltr', 'right' if
'direction' is 'rtl'.
The value for 'text-align' is not part of CSS 2.1.
For 'text-align', specified that 'justify' is treated as the initial
value when computed value of 'white-space' is 'pre' or 'pre-line'.
C.2.104 Section 16.3.1 Underlining, over lining, striking, and blinking
More precisely defined what boxes are affected by text decorations
specified on a given element.
Specified that underlines, overlines, and line-throughs apply ony to
text.
Specified that an underline, overline, or line-through applied across
a line must be at a constant vertical position and with a constant
thickness across the entire line.
Specified how text decorations are affected by relative positioning on
descendants.
User agents are now allowed to recognize the 'blink' value but not
blink, whereas before they were required to ignore the 'blink' value
if they chose not to support blinking text.
Added text to allow older UAs to conform to this section if they
follow CSS2's 'text-decoration' requirements but not the additional
requirements in CSS2.1.
C.2.105 Section 16.4 Letter and word spacing
Support for the various values of 'letter-spacing' and 'word-spacing'
is no longer optional.
Specified that word spacing affects each space, non-breaking space,
and ideographic space left in the text after white space processing
rules have been applied.
C.2.106 Section 16.5 Capitalization
UAs are no longer allowed to not transform characters for which there
is an appropriate transformation but which are outside of Latin-1.
C.2.107 Section 16.6 Whitespace
The 'white-space' property now applies to all elements, not just to
block-level elements.
"\A" in generated content no longer forces a break for 'normal' and
'nowrap' values of 'white-space'.
Specified that the CSS white space processing model assumes all
newlines have been normalized to line feeds.
Added section 16.6.1 to precisely define white space handling.
Added section 16.6.3 to specify handling of control and combining
characters.
C.2.108 Chapter 17 Tables
Moved all discussion of aural rendering and related properties to
Appendix A.
Updated prose to use the terms "specified", "computed" and "used" as
appropriate when referencing values. (See changes to section 6.1.)
C.2.109 17.2 The CSS table model
Defined handling of multiple 'table-header-group' and
'table-footer-group' elements.
UAs are no longer allowed to ignore the table display values on
arbitrary HTML elements, only on HTML table elements.
C.2.110 Section 17.2.1 Anonymous table objects
Changed rules so that internal table elements without an enclosing
'table' or 'inline-table' box generate an anonymous 'inline-table'
rather than an anonymous 'table' when inside a "display: inline"
parent element.
The anonymous table object rules now treat anonymous boxes as equal to
elements' boxes. Replaced several instances of the term "element" with
"box", removed several instances of "(in the document tree)" and
clarified that anonymous boxes generated in earlier rules are part of
the input to later rules. Also replaced the term "object" with "box",
as is used throughout the rest of the specification.
HTML UAs are no longer exempt from the anonymous box generation rules.
C.2.111 Section 17.4 Tables in the visual formatting model
The relationship of the caption box, table box, and outer anonymous
table box has been changed as follows:
* The margins of the table box now apply to the outer (anonymous)
table box that encloses both the table box and the caption(s), not
to the inner table box.
* The width of the anonymous box is now equal to the border-box
width of the table box inside it instead of adapting to the widths
and positions of both the table box and its captions.
C.2.112 Section 17.4.1 Caption position and alignment
The 'left' and 'right' values on 'caption-side' have been removed.
C.2.113 Section 17.5 Visual layout of table contents
Changed rule 5 in grid layout rules to allow overlapping of table
cells instead of leaving skipping a gap in the grid to avoid overlap.
C.2.114 Section 17.5.1 Table layers and transparency
In point 6, changed 'These "empty" cells are transparent' to:
If the value of their 'empty-cells' property is 'hide' these
"empty" cells are transparent through the cell, row, row group,
column, and column group backgrounds, letting the table background
show through.
C.2.115 Section 17.5.2.1 Fixed table layout
Specified that in fixed table layout, extra columns in rows after the
first must not be rendered.
C.2.116 Section 17.5.2.2 Automatic table layout
Restricted inputs to the table layout algorithm for 'table-layout:
auto', whether or not the algorithm described in this section is used,
to the width of the containing block and the content of, and any CSS
properties set on, the table and any of its descendants.
Added rule 4 to include the column group's width in the algorithm for
determining column widths.
C.2.117 Section 17.5.3 Table height algorithms
The 'height' property on tables is now treated as a mininimum height;
the UA no longer has the option of using 'height' to constrain the
size of the table to be smaller than its contents.
Percentage heights on table cells, rows, and row groups now compute to
'auto'.
The baseline of a cell is now defined much more precisely.
Defined the baseline of a row with no baseline-aligned cells.
C.2.118 17.5.4 Horizontal alignment in a column
The value for 'text-align' is not part of CSS 2.1.
C.2.119 Section 17.6 Borders
Several popular browsers assume an initial value for 'border-collapse'
of 'separate' rather than 'collapse' or exhibit behavior that is close
to that value, even if they do not actually implement the CSS table
model. 'Separate' is now the initial value.
C.2.120 Section 17.6.1 The separated borders model
Specified the effect of padding on the table element.
Specified which parts of the table are included in the width
measurement.
C.2.121 Section 17.6.1.1 Borders and Backgrounds around empty cells
Refined definition of "empty" when used as a condition for the
'empty-cells' property so that it is not triggered when the cell
includes any child elements, even if they are empty.
The 'empty-cells' property now hides both borders and backgrounds, not
just borders.
Changed behavior of a row when it collapses due to 'empty-cells': it
is no longer treated as "display: none". Instead it is given zero
height and its associated border-spacing is eliminated.
C.2.122 Section 17.6.2 The collapsing border model
The outer half of the table borders no longer lie in the margin area.
Specified which part of the table is considered the border are in the
collapsed borders model and how its width is calculated. The edges of
the box in which the table background is painted is, however left
explicitly undefined.
C.2.123 Section 17.6.2.1 Border conflict resolution
Defined in rule 4 what happens when two elements of the same type
conflict and their borders have the same width and style.
C.2.124 Section 18.4 Dynamic outlines
Position of outline with respect to the border edge is now only
suggested, not required.
Conformant UAs are now allowed to ignore the 'invert' value. In such
UAs the initial value of 'outline-color' is the value of the 'color'
property.
C.2.125 Chapter 12 Generated content, automatic numbering, and lists
The 'marker' value for 'display' does not exist in CSS 2.1
C.2.126 Appendix A. Aural style sheets
Chapter 19 on aural style sheets has become appendix A and is not
normative in CSS 2.1. Related units (deg, grad, rad, ms, s, Hz, kHz)
are also moved to this appendix, as is the 'speak-header' property
from the "tables" chapter and other notes on aural table rendering.
The 'aural' media type is deprecated in favor of the new 'speech'
media type.
C.2.127 Appendix A Section 5 Pause properties
Changed the initial value of 'pause-before' and 'pause-after' to be 0
instead of UA-defined.
A note has been added to this section (formerly 19.4) about the change
in position and behavior of pauses in CSS3 Speech compared to this
appendix.
C.2.128 Appendix A Section 6 Cue properties
This section (formerly Section 19.5) now specifies the placement of
cues and pauses with respect to the :before and :after
pseudo-elements.
C.2.129 Appendix A Section 7 Mixing properties
The keywords 'mix' and 'repeat' may now appear in either order.
C.2.130 Appendix B Bibliography
Various references in Appendix B (formerly Appendix E) have been
updated as appropriate.
Switched [CSS1] from Normative to Informative.
Updated URI reference from [RFC1808] and the
draft-fielding-uri-syntax-01.txt to [RFC3986].
Updated HTTP reference from [RFC2068] to [RFC2616].
Removed normative references to [IANA] and [ICC32].
Added normative references to [ICC42], [RFC3986], [RFC2070], [UAAG10].
Added informative references to CSS2, CSS3 Color, CSS3 Lists,
Selectors, CSS3 Speech, DOM 3 Core, MathML 2, P3P, RFC1630, SVG 1.1,
XHTML 1, XML ID, and XML Namespaces.
Removed informative references to [ISO10179] (DSSSL), [INFINIFONT],
[ISO9899] (C), [MONOTYPE], [NEGOT], [OPENTYPE], [PANOSE], [PANOSE2],
[POSTSCRIPT], [RFC1866] (HTML 2), [RFC1942] (HTML Tables),
[TRUETYPEGX], [W3CStyle].
Updated language tags references from [RFC1766] to [3066].
C.2.131 Other
The former informative appendix C, "Implementation and performance
notes for fonts," is left out of CSS 2.1.
C.3 Errors
C.3.1 Shorthand properties
Shorthand properties take a list of subproperty values or the value
'inherit'. One cannot mix 'inherit' with other subproperty values as
it would not be possible to specify the subproperty to which 'inherit'
applied. The definitions of a number of shorthand properties did not
enforce this rule: 'border-top', 'border-right', 'border-bottom',
'border-left', 'border', 'background', 'font', 'list-style', 'cue',
and 'outline'.
C.3.2 Applies to
The "applies to" line of many property definitions has been made more
accurate by excluding or including table display types where
appropriate.
C.3.3 Section 4.1.1 (and G2)
DELIM should not have included single or double quote. Refer also to
section 4.1.6 on strings, which must have matching single or double
quotes around them.
Removed "A-Z" from the "nmchar" token: as CSS is case insensitive
anyway, it was redundant.
Corrected "unicode" macro to treat CRLF as a single character.
Corrected "block" production to allow white space between
declarations.
In the rule for "any" (in the core syntax), corrected "FUNCTION" to
"FUNCTION any* ')'".
C.3.4 Section 4.1.3 Characters and case
Corrected third paragraph to say that an '@import' rule can only be
preceeded by an '@charset' rule or other '@import' rules.
C.3.5 Section 4.3 (Double sign problem)
Several values described in subsections of this section incorrectly
allowed two "+" or "-" signs at their beginnings.
C.3.6 Section 4.3.2 Lengths
Fixed double sign error in definition of . ( already
has a sign.)
Corrected the suggested reference pixel to be based on a [INS: 96 dpi
:INS] device, not 90 dpi. The visual angle is thus about [INS: 0.0213
degrees :INS] instead of 0.0227, and a pixel at arm's length is about
[INS: 0.26 mm :INS] instead of 0.28
Corrected last sentence to refer to a unsupported used length, not an
unsupported specified length.
C.3.7 Section 4.3.3 Percentages
Fixed double sign error in definition of